Let's see, my top ten things to do... I tend towards eating and kanoodling with my wife, so this may skew differently then other's tastes.

in no particular order:

Hike at Red Rock Canyon national park
Eat at Isla mexican restaurant at Treasure Island (excellent food!)
Watch the Dancing Waters at Bellagio
see the new Mirage Volcano
Visit the Luxor before they take down all the Egyptian statues in the front lobby
Eat lunch at Verandah at the Four Seasons on the Mandalay Bay property. (get served like the rich! also superb food)
go to the top of the Stratosphere tower for sunset
visit the Bellagio's interior garden
go to the Dueling Piano show at NYNY (there are many, but this one's my favorite)
See the animal exhibits at Mirage, Mandalay Bay, and MGM Grand (ok, that's three)