Family Fun Magazine had a good article in this past month's issue which got us started. (I don't think this month's activities are on their website yet. I'll try to link to the specific ideas when Family Fun updates their site.)
1. We did some digging in the yard to look for worms and we tested Family Fun's idea of how to get worms to come up to the surface.
2. We used materials that we had around the house to study the underwater critters at our nearby lake. We also tested the pH levels.
Since we had recently read "Sign of the Beaver" for history, we also tried to leave "signs" to show ourselves the way back (even though we already knew the way).
The kids and I really enjoyed our time!