Well it depends on what you like to do. Not too much to add but I will second the free little show in front of TI http://www.treasureisland.com/shows/sirens_of_ti.php and the Volcano at Mirage and the Fountains at Bellagio. You can watch the fountains of Bellagio from the top of the Eiffel tower. You can see the strip and surrounds way up high (and grab a drink or ride some rides) on top of the Stratosphere Tower. The conservatory and glass ceiling at Bellagio is neat to see as well. http://0.tqn.com/d/govegas/1/0/m/Q/1/chihulybell2.jpg
For the paid shows, besides the various Cirque ones (i.e. O, Mystere, Zumanity) and La Reve, there is Rock of Ages, and my own favorite, Absinthe.
There is the Freemont Street Experience, downtown. You can also get your picture taken with 1mil in cash for free at Binion's (formerly Horseshoe) http://www.binions.com/million_dollar_photo.php . If you are into it, you can go eat downtown at the Heart Attack Grill http://www.heartattackgrill.com/ where you eat free if you are 350lbs or more! Finally a place I have always wanted to check out but haven't yet... drink and drag http://www.drinkanddrag.com/index.php looks like a lot of fun.
Perhaps you want to get a drink and watch the Lake of Dreams show at Wynn Las Vegas at Parasol Up or Parasol Down. http://www.wynnlasvegas.com/Restaurants/BarLounges/ParasolDown
Away from the casino areas, there is the National Pinball Hall of Fame which is fun if you like pinball. http://www.pinballmuseum.org/
And my favorite website for info : http://www.lasvegasadvisor.com/ Check out the top 10 list for some deals or even learn about transportation, pools, or gay Las Vegas.
Enjoy your trip.