Are you backpacking for 3 days or doing day hikes 3 days in a row? You should figure out your gear then find a bag that fits it. I use an MEC 6L for light hikes, Deuter Speedlite 20L for long summer hikes, MEC 30L for winter, and a Gregory Z 75 for backpacking (it's too big). I personally really like Gregory's bags for hiking and would go that direction if I was going to buy a new hiking bag.

A 3 day hike is out of the carry range of most hydration-specific backpacks... that is to say, packs on the market that include the tube and bladder. Find a 45-55 liter bag with adjustability so you can dial in the height/fit, and add a hydration pack from Hydrapak (the company who does the built-in stuff for Osprey.

Now, for pack recs:

Osprey Exos 48, insanely light and comfortable thanks to their suspended back panel.

Backpack Light 50 from MyTrail. Small Colorado company run by backpacking freaks, this pack has been dialed for a long time and is fairly affordable.

Black Diamond mission 45. Black Diamond is a no-nonsense company (by and for climbers). They use hardcore materials that can last a lifetime!

Don’t forget a good filter! Grayl is easy to use and is available at a great price

Osprey or Camalbak, both have awesome customer service.

A "hydration pack" is pretty much all packs nowadays, so your pack size is one question (day hikes, overnights, multi nights out), and then once you have the capacity you want down (I'd go 15-35L capacity for your distance, this will allow for you to expand for some overnights if you want), narrow this further to packs with a pocket for a hydration bladder and holes for feeding the tube through.

Ultimately your pack choice is going to be a personal thing based on in-person fit, comfort here is key.

Many packs will come with a bladder and drinking tube, but you can also buy these in various sizes quite easily. A 2L bladder would be enough for me on that distance but how much water you drink is a personal thing.

TLDR: Go shop for a day pack that is comfortable for you, look for one that has a pocket for a hydration bladder and you might even find one that comes with the bladder. otherwise they're not terribly expensive to pick up! Platypus, Osprey, and Camelback are very reliable bladder brands and make decent packs too.

For day hikes I carry a North Face bag similar to the Klamath. 2L hydration, enough room for emergency supplies/first aid kit, a snack, and even my wind jacket if I pack it down. Mine's not a Klamath, but I've had it for about 15 years and has been a constant companion.

Do you mean just the bladder, or the entire pack?

I use the Osprey Raven 14 (womens). It's a mountain biking hydration backpack BUT I use it for day hiking (5-15mi) and not MTB. It comes with a 2.5L hydration bladder insert and has enough room for personal items and snacks and a jacket plus some camera gear.

There is a men's version... if U need that. Also just hydration packs in the MTB section

I don't know if you got something but I'd still suggest checking out this Water Buffalo. It's a pretty good value pack. I use it all the time and never had problems.

I have THIS one from amazon and have really liked it. There is plenty of storage and for $50 its hard to beat!

Ultimate Direction by far the best pack I’ve had and it has a pocket for a bladder

Pretty much all packs are bladder compatible. 

Get a bigger CamelBak... I have a nice Gregory and a different Gregory model before but the last CamelBak I had was way more comfortable but these ones were bigger which I needed for my explorations.

Backpacks are a personal choice because that comes down to fit and capacity which I can’t really comment on. I will warn you that your girlfriend’s bust size will impact comfort, especially for the 1.5-10 liter capacities since those will tend to be more form fitting and “spandexy” to reduce movement of the hydration bladder. However, the hydration bladder I can comment on. My favorite is Osprey because it has a quick disconnect on the hose which means you’re not screwing around with having to in-thread the line and mouthpiece through the shoulder strap every time you want to refill it.

Pro-tip: when you want to dry out the hydration bladder take the grey plastic slide rail that is used to close the mouth of the bladder and I see it into the opening to hold it open. Hang upside down to dry.

I have a Gregory Swift H2O backpack and I really love it. They are a company that only make backpacks, so their designs are very spot on. They design for women as well so the straps don’t go over your boobs. My pack is a 25L, but they have smaller ones. It is very lightweight and comes with a nice hydration pack that is pretty functional. Also, they are not crazy expensive, wich makes it a good alternative.

I would stay away from water bladders if you’re going to be in sub freezing conditions because it freezes and could break the bag.

Patagonia 25L Black Hole

Take her to the store. Every brand has different straps etc that hit each persons body differently. Both do you will be much happier with the purchase if you get it right the first time. I’ve learned I hate pockets in hip belt when not on a long multiple day trek - but that’s just my irritation not everyone thinks the same. On a day out I don’t go over 15L even if I have to carry emergency overnight gear.

Durability, storage, comfort; Osprey, Kuiu, Mystery Ranch

Volume sizes; It depends on what you’re doing, and what you’re packing. Generally 15-30l for daypacks, 30-60 for multi day trips, 60+ for expedition.

Feature recommendations depend on what you’re doing. All major pack companies come with hydration bladder compatibility and most are made with lightweight materials.

Make sure you get someone to help you fit the proper sized pack, not all brands/models fit the same. Put some weight in it at the store and wear it around for a while. Educate yourself on how to adjust the pack properly as well as weight distribution while packing it.

My go to daypacks are; Arc’teryx Brize 32 for EDC, Kuiu Pro 2300 for outdoors specific.

My larger pack for SAR is a Savotta Jaakaari L but I don’t think it’s what you’re looking for

Osprey and Gregory make sturdy and comfortable packs but they are a tad heavy before you get into UL designs.

I’ve got a deuter trail 23 and it’s pretty solid. I wish I had a larger version. I’ve also got a Gregor Zulu 40, it’s not as nice as the deuter

I've got Deuter Aircontact Lite. Awesome backpack. Very comfy loaded and in warm climates. I often do multi-day hikes. I love good back ventilation, that's why most UL backpacks are not for me.

Do you know the weight you want to carry? That would impact your choice.

I loved my Gregory before I moved to UL.

Smart water bottles with the one bottle hydration tube connector. Best of both worlds! Easily connects to my Sawyer squeeze and a cnoc bag.

huge fan of the Gregory 3d. Haven't tried many others but it fits well in my pack, and the shape is easy to pack around, as well as every piece will come apart to clean and dry it, which is great because im paranoid

Osprey hydraulics. Easy to put in and out Of my osprey pack

I have the Gregory 3D 3L, in a Gregory pack and really like it. The handle makes it easy to fill and the closure is simple. The tube disconnects for easy removal and it also has a hook for hanging upside when trying to dry it out.

Gregory 3D is awesome. I use them in every pack.

I have a lot of packs in my closet, and I’ve donated a lot more to friends. My USWE packs are the ones i reach for every time.

If I use a backpack style hydration pack (seldom), it is an Osprey Syncro 5. It offers 2.5L of fluid capacity. Supposedly 5L of storage capacity, but I think that is reduced by the bladder itself. It is definitely enough room for tools, small food items, etc.

Anyway, it is pretty minimalist, which I actually like. It also has a bit of a frame so that the pack itself is held away from your back, which keeps things a bit cooler.

I love the helmet retention feature, which lets you attach your helmet to the back of the pack. Very handy for packing your car up.

Number one dislike is that the bite valve comes off of the magnet attachment constantly and swings around. I usually end up tucking it under the shoulder strap on my chest. I think this is a very common problem with these things.

It's also kind of expensive, so there's that.

Osprey whore here. One of the big pros about Osprey is their warranty "all mighty guarantee".

Osprey is committed to ensuring our products withstand the test of time. Torn piece of fabric? Worn-out buckle? Broken zipper? Whatever the damage, we'll repair or replace it at no cost—whether your bag was produced in 1974 or yesterday.

Just went through their warranty system for my raptor 10 bought in 2016 and they are sending me a brand new pack Thursday. Mine was a busted zipper for the hydration section of the pack.

That’s a stout pack. Very nice.

I have an EVOC six litre pack with a two litre bladder in. Holds enough crap and liquid to get me through the ride.

I have multiples each of Osprey, Camel Back, Gregory and Ozark Trails hydration bladders along with their various related packs.

Beyond bite valve preferences they are all long lived and do their job as they should and I would be hard pressed to find a reason to buy the most expensive ones over the cheapest ones. At least in terms of bladders. If your question extends to the packs they will occupy then the question becomes more complicated and is an equation involving “cost, features, and comfort” with each variable weighted differently by how much you value it.

I love my Platypus Big Zip. I have an older version with the tube connection on the bottom of the bag. They revamped the design a bit though.

platypus hoser bladders are good n cheap. as for the backpacks, there are so many that will accept a bladder, just pick what size and color you want.

the zip bladders are easier to clean, but the regular hoser model works well for me. as for bite valves and stop valves, camelbak is king.

Liquid IV. Not the cheapest but it’s been my choice for about the last 3000 miles and I’m not dead so it must be doin somethin right

I take Cure electrolyte packs when doing my long runs. Tastes so good and has close to 1:1 sodium and potassium

Nuun

Cheapest/ best source is to just buy Morton brand lite salt. It’s part sodium part potassium. You get like 1000 servings for 2$. Everything else has very minimal potassium

LMNT