Every time I'm in Vegas, I always make it a point to visit the Bellagio fountains. For some reason, it's just relaxing to watch the wonderfully choreographed fountains dance and twirl in complete unison to the chosen song of the moment.
Personally, I prefer to watch the fountains at night since there are also lighting effects, albeit tiny, used in the show. Also, I believe after 8pm, the shows are every 15 minutes so you can enjoy this wonderful attraction all the more!
Bottom line: A must see attraction for every Vegas traveler!