Spend some time at the Bellagio watching the Fountains of Bellagio, the best free show in town, and walk through the conservatory, another amazing wonder of Las Vegas.
Also, don't miss the "Big Shot" at Stratosphere and the roller coaster at NY, NY. Check out "Rain", the dance club at the Palms and have dinner at the Rio World Buffet (although it has gone downhill a bit in the past few years)
I would second the motion of seeing Blue Man Group. It's an awesome show, an totally unique. I was in Vegas two weeks ago and hung out with the girl that's the stage manager for BMG at the Luxor. Good times and good show.
, make sure you get a Double-Double at the In-n-out burger on Tropicana west of I-15, they've got the best burgers in town!