the bellagio has an awesome fountain show. You can see it from right in front, your hotel room if you are lucky enough to stay their, from atop the Eiffle tower (which is really cool) or from the walkway across the street like this shot. they are all corregraphed to music and gorgeous. and it is free.
vegas has a lot of free shows like the Mirage's volcano, MGM has a lion exhibit. Ceasers has an atlantis sculpture that comes to life, Excaliber has a dragon slayer,Venetion has some fountains that go off in the canal shop area and Treasure Island or TI as it is now called has the Siren Pirate ship show. It use to be just pirates but they had to "sex" it up for sin city and in my opinion it is not very good. Below is a shot of the firet that gets started as one ship fires at the other.
we ended up getting to it later then we wanted to because we thought it started at 9 but it started at 830 oh well just as well.