If you are ever in the Los Angeles area, the Le Brea Tar pits is a wonderful resource. http://www.tarpits.org/
It's the largest active excavation site, with over 1 million bones being recovered since 1906. The museum is open year round, however they do the excavating during the summer months. Their most complete skeleton, that of a mammoth, was found only a few years ago (I believe in 2004). There is a fee to enter the museum, but the outdoor grounds (where you can view the asphalt bubbling up and the excavation sites)is free. If you check the website, there is a free day once a month.
We have goneseveral times, and my son still asks to go back.