My top picks would be:
1. The Conservatory at Bellagio, their ever changing unbelievable floral display; as Teach8 stated, we also do this EVERY trip.
2. The Grand Canal Shoppes at Venetian; beautiful indoor water canals and wonderful murals on the walls and ceilings in the Venetian lobby and entrances as well.
3. The Cosmopolitan; the chandelier bar, as well as all the other glass within the casino is breathtaking.
4. The Silverton Casino; the 117,000 salt water fish aquarium at their main entrance is incredible. The Bass Pro Shop located there is pretty interesting as well.....and lastly,
5. The BIG ELVIS show , a free show, afternoons starting at 3:00 in the lounge at Bill's Gambling Hall. If you close your eyes you will think you are truly listening to Elvis sing !
These are my favorites, but I also think that no matter what you do in Las Vegas, you will surely have a great time !