Did you know that your city probably offers a ton of free admission days to local museums, zoo, gardens etc.? Here's a list of Denver's free days. If you don't live in Denver I bet if you google "free days (your city) 2009" you will find the same sort of list. The only catch is very rarely are they on Saturdays...which makes it hard for the working family.
Blaine and I totally took advantage of free days last year and went to the Museum of Nature & Science. As you can see we had fun...for free!