I thought Manoa falls was beautiful! Anyone who happens to be a fan of Lost or Jurassic Park will love it even more because pieces of each were filmed here. Walking up the trail is pretty cool because you can envision a T-Rex to suddenly come stomping out of the trees, or as a Lost fan, you feel like you're on a mission across the island.
Websites will advertise that there are posters of Lost filming scenes posted inside the main building, which I pictured to be actual posters. What is actually hanging in the main building is a poster board inside the snack bar that has cut out pictures of the cast on filming location and scenes that were filmed there. It is still very cool to see, just not what I was expecting, and its easy to miss it unless you're looking to find it.
Parking is $5 so have cash when you arrive and shoes that you won't mind getting a little dirty. The first time we were there the trail was not muddy at all, just very cool temperature. The next time we showed up to hike it had just rained, and was continuing to rain, and the trail got very muddy. We were able to finish without being too much of a mess (unlike some people) and I think they would recommend not hiking in flip flops or business dress. (I'm not sure why some people were hiking this trail to begin with?)
The trail itself is only about a mile round trip so it's not very strenuous, though there are some stairs as you get more steep. The falls itself are very beautiful and take note of the "do not cross" signs that everybody ignores- everyone goes under the rope to take pictures by the waterfall but don't fall on the rocks.