I used to use a fanny pack (don't really like the look, but they're light and don't allow you to overpack) but last time when went I used a small backpack instead. I really liked the smaller backpack, but you have to make sure it's small! The tendency to overpack a "normal" sized pack is too great when you've got the room.
This summer, we're each traveling with our own Camelbaks as keeping hydrated is a key goal and I don't want to lug around half-frozen water bottles for everyone all day. The kids have Camelbaks with very little room for storage (enough for a poncho), but mine has 2 extra pockets so I'll also be able to carry my sunglasses, a small digital camera, the park passes, 1 bottle of sunscreen, my poncho, and antibacterial hand wipes.
Bottom line, keep it light! Even 5 lbs can feel like 10 when you're carrying it around all day!