I did Vegas without the car, there is the Deuce bus that runs along the strip, all the way downtown, something like $3 one way, $7 all day or just $15 for three days. I never really had to wait super long for a bus, though during some of the busier times I did have to wait for a following bus because the first one was full. Seemed a good trade off for not having to worry about a car, gas, valets, insurance, etc. I'm not much of a drinker, but I think not having to drive would be a big plus...

I did a Hoover Dam/Grand Canyon tour, they had a bus pick me up from in front of the hotel. If you plan on doing just the Hoover Dam, I think the bus tours will be a good option. If you plan on doing the Grand Canyon as well, you might be better off with your own car and spending the time as you want at each destination. Since we were heading to the Grand Canyon, we only stopped at the Hoover Dam for 15 minutes. The tours that just go to Hoover Dam, seem to be there longer for you to take in everything.