The Volcano at The Mirage
http://www.mirage.com/attractions/volcano.aspx
For something fun and free to do in Las Vegas, check out The Volcano at the Mirage Hotel & Casino. It's located directly in front of the Hotel, you can't miss it. The Volcano comes to life and erupts hourly, on the hour each night weather permitting. The eruption lasts for about 5 minutes or so and is very cool to watch.