Rent a car and get to the Dam for the first tour about 9am. Take the full tour with the tunnels, it's pretty cool. Then after your done you can drive over to Red Rock national park and do the 13 mile loop drive, also very pretty.
I found renting a car for the day from who ever has a car rental counter at the hotel is the same price as paying for 2 tickets on a bus tour. Make sure to rent car online before you leave home for the best price. Just call hotel and see who has a counter there.