I went through a number of day packs in the past fifteen years.  Usually a zipper goes.  I've had an Arcteryx day pack for about 6 years that I bought for it's heavy duty zippers.  It has held up well to a lot of abuses and can hold a surprising amount of gear.
Last season I used the BWJ Guide pack for the entire season.  Relatively expensive at $100, but I believe it's going to be a long term keeper.  Tough, spacious and comfortable.  It has two large side pouches with serious two way zippers and a zippered front pouch, as well.  I cut an old ridgerest pad in half and trimmed one half to make a 'liner' for the main body of the pack.  Pulls out to be a short camp pad and protects the contents (and your back--the pack has no padding).  It always has the first aid and repair kits, a fleece jacket and spare socks in a little drybag (this is also my pillow), tackle and gear (fillet knife, pliers, stringer, etc) and all the miscellaneous odds and ends (extra compass, handwarmers, space blanket, a couple of short lines, lighters, firestarter, etc) in it and has plenty of room for water bottles, lunch sack, cook kit, etc.
I find it handy, as a solo paddler, to have a pack like this along.  It goes on all the day trips, working as ballast and stowage.  I'm a bit ocd about my systems and placement of gear, I like to know where something (especially first aid, etc) is, at all times when I'm out there.  Most of the gear has it's place.  I hate wasting time searching for something.  But enough about me...