Nature in Nebraska

I want to do something fun and nature-ific this summer.....but here's the catch: it has to be in Nebraska and 3 hrs or less away from my house. We're going to try camping in our backyard for starters, but if all goes well, I'd really like to venture out! Does anyone have anywhere they'd recommend? More specifically, if you have small children and you've found a hot spot to hike or camp, could you please share? I'd really love for Emily to see a waterfall, but I think the Niobrara is the closest thing around here (and 7 hrs away is just a bit too much for us right now). Anyhow, I'd love any feedback.