There are county, regional, state, and federal parks in the metropolitan area. Check your county site (not listed here) for parks, classes, camps, and fun events.
Virginia State Parks http://www.dcr.virginia.gov/state_parks/index.shtml
Maryland State Parks http://www.dnr.state.md.us/publiclands/
DC Parks and Recreation http://dpr.dc.gov/dpr/site/default.asp
National Park Service http://www.nps.gov/
Local hikes: http://www.localhikes.com/MSA/MSA_8872.asp
PUBLIC LIBRARIES
You will LOVE these after having no books overseas! Check what is available in your county--or nearby (many counties have reciprocal agreements). Most offer children's programs, and some sponsor community activities, including book clubs, Spanish conversation groups, or ESL classes. With a valid card you can use the library website to reserve books, do research, or even learn a foreign language!
MUSEUMS
Many are FREE!
CONCERTS
In addition to the many top-rate classical and popular concerts in the area, there are daily free concerts at the Kennedy Center (handily close to the Main State Department building) - no tickets required
http://www.kennedy-center.org/programs/millennium/schedule.html.
Many local choral, instrumental, and theater groups offer excellent performances. For more information, check your local paper or the Friday Washington Post Weekend section listings.
There are free outdoor concerts in dozens of venues over the summer; check your local paper, county parks website, or The Washington Post for listings.
THEATER
Enjoy anything from a local production to a Broadway show. You will find listings in newspapers or identify an interesting company through the League of Washington Theatres.