Tree Adventure at Arbor Day Farm is really fun and definitely worth the drive to Nebraska City. Treetop Village has 11 treehouses connected by netted walkways. There's also a hiking trail and natural playscape area.
I recommend arriving early. We came when they opened and it was really nice being in the Treetop Village on our own. It got quite crowded later in the morning, and not everyone respected that there's a one-way path through the treetops.
The WonderNet area reminds me of a trampoline and is a fun area for kids (and adults) to jump.
Pants are recommended due to this 50-foot rope slide, which my kids and I both enjoyed. However, there's an alternate way down, if you prefer to avoid the slide.
There was only one long narrow walkway that scared my kids, and I have to admit I didn't want to look down when crossing. It definitely was easier to cross the second time though. You can go through the Treetop Village as many times as you like. In total, we spent 3 hours at Tree Adventure.