Be sure to check out all the special events & promotions that are being offered at the parks, beyond the normal activities. You could stumble upon something really fun.
After looking at National Parks in our state & neighboring states, I saw that the Effigy Mounds are only a few hours away and are now on my list of weekend vacations for our family (some time in the future.)
If you can't get to a National Park during that week, there are a couple other days in the future you should be able to get free admission for:
September 25, 2010 (Public Lands Day)
November 11, 2010 (Veterans Day)