I use a Camelbak Honcho for day hiking and day skiing excursions. The compression/shovel flap really comes in handy. The Honcho fits me well, and offers plenty of support for a load.
I use an REI Jet Pack for commuting to and from work. It handles two Nalgene bottles, food for the day, a first aid kit, and a small compliment of just-in-case stuff. It is a simple and light pack, and doesn't offer a ton of support for the load.
If this will be a serious day pack for day hikes, make sure you get something that fits you well. I've seen folks who buy packs based on looks or features, and then suffer when they need to carry a full day hike load. I'll admit I did that once, but never will again.