I also would vote for the dam tour. Also, my girlfriend and I took a tour to the Grand Canyon (we took the tour that included the "flying horseshoe" thingie on the indian reservation that hangs out over the canyon) on Grey Line Tours last February. It was a great trip, but might be a bit pricy. I remember spending $100 or thereabouts, but I can't remember if that was for two tickets or one. If you check out thier website, you can (or used to be able to...) get tickets for half-price if you get them in advance via the website.

Check out the las vegas advisor website too. Plenty of great ideas and coupon specials there.