there are various ways of doing rockets, that is always fun. alka seltzer rockets or water-powered rockets are both good alternatives.

nature: some kind of collection or observation exercise. Have them find natural items in every color of the rainbow, or do a sensory hunt (find things with different feels, or that make different sounds).  (maybe have them write it down instead of actually picking things up if LNT is an issue). If you want to do something that's a take-home you can have them plant seeds in small containers that they can decorate (egg cartons, etc).

When we did owl pellets last time we tied it in to the food chain and had them work on their wildlife conservation beltloop in conjunction with it. I had a poster with velcro dots on it, the kids could velcro on the pictures of the animals and plants to show who eats and gets eaten by what.