The Fountains of Bellagio; it's a cool free show just be prepared to fight a crowd to get a good vantage point. We started off inside but realized we couldn't hear the music or feel the mist and on a hot day the cool and refreshing mist is rewarding.
The entire show lasts about 10-15 mins and if you time it just right it's a free show you will be telling your friends and family about once you get home. I've seen the show twice now first time as a single male with a group of guys and we weren't impressed with the show but if you bring a date and you get a good vantage point it turns into a very nice and semi romantic experience.