In a lot of areas, museums offer one free admission day a month. This is a great opportunity to get to take the kids to some pretty interesting exhibits.

In my area, for example, there are a lot of museums in Balboa Park. They have Free Tuesdays, where each of the first four Tuesdays of the month different museums have a free admission day. Some are for county residents only, while others will let anyone in free.

You can still make donations to the museums if you like, of course.