For day hiking, you have two choices: the traditional pack with shoulder straps or the waist pack, sometimes called a fanny pack. Each has its own merits. The biggest advantage of the traditional pack is size; you can get a lot more in it. However, if you have shoulder or back problems or want a cooler back, especially in summer, check out the fanny packs.
Fanny packs are available in a range of sizes and designs. Figure out how much paraphernalia you want to take with you (see list below) and look for a pack that will accommodate it. External straps are a great idea because they allow you to lash extra stuff to the outside. A strong, substantial waist belt and buckle are also important.
It's handy to have your water bottle easily accessible, rather than having to stop and rummage through your pack every time you want a drink. Some packs are designed with an external pocket for your water bottle, or you can add a water bottle holder to your waist belt with a Velcro strap. Some camera pouches are designed to fit a waist band, and you can also suspend a small pair of binoculars or a small first-aid kit.