I love the Black Canyon River Adventures. I am a local and have done this tour twice. Both times were a really neat experience. You really get to go out and experience a different side of Nevada/Arizona.
The tour begins at the base of Hoover Dam. It is so amazing to look up at the dam, instead of over the side like most people. It is really cool to have a different point of view of such an awesome work of engineering and manpower. You then get to float down the Colorado river, which is 55 degrees year round, and see fish swimming beneath your feet, big horn sheep on the cliffs, and all sorts of interesting birds flying just above your head. You also get to go see natural hot springs coming right out of the mountain sides, an emerald cave with the clearest water I have ever seen, and just get in touch with nature, but without too much work on your part.
On top of the wonderful float ride down the river, is your tour guide. These guys know everything about the area and are pretty funny too. You will learn some neat stuff and laugh a lot too. They totally make your day.
Overall, I think this is a total must do if you are visiting town or live here. Get out and do something different!