OK, its not the Bellagio fountains, I could watch those all day, but the Mirage Volcano is a fun, quick, free must see on the Vegas strip.
The Volcano erupts every 15 minutes in the evening. It is a spectacular show of fire balls and water made to look like lava all choreographed to music.
I like it because it is quick too, plus you can see it from the strip or the walkways around the Mirage. It does get busy around eruption time, so find a spot and enjoy the show!