Must Visits:

Fremont Street (after dark)
Fountains of Bellagio
Stratosphere Tower
The Venetian (shops)
The Grand Canyon
Siegfried and Roys Garden (can't remember the name of it)

If you have time:

MGM Grand and the Rainforest Cafe
The Legends show in the Imperial Palace
Harrahs has some top top live bands on

Don't bother with:

Circus Circus
The one that looks like an egyptian pyramid

Of course there is loads more. How long are you going for? Have you booked? If you are on a budget Harrahs is a great hotel in a superb location. If you aren't on a budget stay in the Bellagio or if you like lazing around a pool, the Mirage has the best swimming pool area you have ever seen!

If you wanted to make it a longer trip you could take a short flight to San Diego, which I would highly recommend for a few days! PM me if you have any more questions on this.