There are typically 7 to 8 Free days at the Denver Museum of Nature & Science in Denver, Colorado. There are only 2 days left this year. So if you are looking for something inexpensive for the family check out the museum on these 2 days.
Free Days are for Colorado residents only. A fee still applies to IMAX films, and Planetarium shows.
Here is a list of there current exhibitions:
Gems & Minerals
North American Indian Cultures