What would you take with you?

We are taking our children "2 & 4" to the Zoo tomorrow for my daughter 4th birthday. We took them last year as well, but our son was still at an age where he would ride in the stroller and we had a diaper bag with us full of all the stuff we would need. Now that he is older and refuses to ride in a stroller I'm not to sure about what to bring. I don't want to under-pack and end up needing something I forgot, and I don't want to over pack for the simple reason I will be the one lugging the bag around all day. We have the little back packs for the kids so they can walk on there own with out running off. Each bag holds a few small things. If you were in my situation what would you bring with you? Keep in mind I don't want a back that weighs 20 lbs.