The Lewis and Clark Visitor Center in Nebraska City is definitely worth a visit if you're in the area. We started inside, checking out the exhibits on all three levels, which are very kid-friendly.
Then we headed outside to look at the keelboat replica, which was built as a movie prop for an IMAX movie.
The hiking trails are short, but worth checking out. We hiked all of them except the Meadow Trail, because it was really hot during our visit.
We ended our visit at the Earth Lodge, which my kids thought was really cool and they couldn't believe people used to live in them.