It doesn't matter if you're a tourist or a long-time local, the fountains at Bellagio are a must see.
The show happens every half hour starting at 3pm and and goes well into the evening. Each half hour has a different song usually something classy (Sinatra), dramatic (Bocelli), romantic (Dion) or fun (Pink Panther). Each song has it's own special water choreography making use of the entire faux lake in front of Bellagio.
Even though the show starts in the afternoon, you must come back at night. The lit up water with the backdrop of the strip might be the most beautiful thing you see in Vegas. For five minutes your mind can take a break from the artificial city and take in this awesome free work of art. And since Bellagio attracts an international crowd, it's really cool to see people of all walks of life come together to appreciate it.
I guess the only thing I would change is maybe try something with more modern tunes. Still, the fountains are more than deserving of five stars.