I kayaked the Colorado River with my brother while in Vegas. It was an awesome trip, and it was put together by Evolution Expeditions -- I can't recommend them more highly. They picked us up at our hotel and dropped us back off in lovely, air conditioned vans with really friendly drivers. Our river guide, Jon, was a great guy, funny, knowledgeable, and a good teacher. If you feel like doing something outdoorsy and adventurous next time you're in Vegas, check them out!