I have an Overboard Waterproof Backpack. its 25 liters with a roll top, and i love it. its good for going to school, the store, or something like boating. obviously its good in the rain. it has large thick shoulder pads which are comfy. it also has chest and waist straps. here is a link for it, but if you look for more then two seconds you can find it for way cheaper. they also make it in a 20L and in different colors, although i prefer black everything.

http://www.thewaterproofstore.com/ob1014blk.html