When in Vegas, definitely go see the Bellagio Fountains. Located pretty much in the middle of the strip, it's an amazing FREE show. It's beautiful whether you see it during the day or at night. Water dancing to classic Frank Sinatra? Sign me up!
The fountains show still starts at 12 pm and runs every 30 minutes, and sometimes more often in the evenings. Just try to get there about 10-15 min early because it can get pretty crowded.