On a day hike, you want a hydration pack that carries plenty of liquid, but also has room for all the gear you need—extra clothes and food, first aid kit, headlamp, sunglasses, and so on.
Osprey’s Manta 30 ($149) is just the ticket. First, it’s an excellent hydration pack, with a 100-fluid-ounce bladder that fits against an anatomically contoured plastic sheet so that it rides comfortably. The bladder has a large mouth for easy cleaning and refill, and the drink valve pivots so you can sip from either side.
The Manta also is an excellent daypack. It has a light alloy frame and contoured shoulder straps so it carries well even when jammed full of stuff. And with 1,600 cubic inches of capacity in the small/medium size, there’s plenty of room for gear.