If you're even remotely considering seeing that piece of trash at TI "The Sirens of TI" keep on walking... toward the Mirage.
I remember the first time I saw this in 2006 - I thought it was pretty cool. As for free shows, I'm not enamored with the Bellagio fountains, nor am I enamored with this "volcano" but I'd hit it up again if I were a tourist - but I can't say that for Bellagio. I'd say that this is the only real "kid friendly" outdoor spectacle on the Strip.
If you're walking the Strip at night, and you happen to be in that area (and you should at some point because it's near the Venetian) check it out.