Kid-friendly things to do in Washington DC that aren't museums

National Zoological Park to see panda bears, Great Falls National Park, VA to see waterfalls (you won't believe how spectacular they are), National Aquarium in Baltimore, International Spy Museum- there is a charge, Old Town Alexandria for shopping, eating, etc., Mount Vernon Mansion-George Washington's house.

Do you have a car? Glen Echo Park is in a suburb north of DC. It has a lovely Carousel (currently runs May through September, so check before going), puppet shows, artisans (including glass blowers), art shows, music and more. There is another nearby park Cabin John Regional Park that has a train. The Clara Barton National Historic Site (founder of the American Red Cross) is also a fascinating stop in the area. Imagination Stage (children's performing arts) may have a show or other event you want to catch. And there is great food of every stripe nearby (adults, children ). Finally, also in proximity is a neighborhood, Kenwood, that has an extensive wonderland of extraordinary cherry blossom trees.  Beautiful in April for a walk or drive. 

Hi Avery - I've spent most of my life in the D.C. area, although I'm currently living in Switzerland.  One place I loved to visit when I was a kid was The National Zoological Park.  Also, if the cherry blossoms happen to be in bloom (a distinct possibility in April), then a paddle boat ride on the Tidal Basin is a must.  Have fun!

I found a really good rundown of outdoor spaces here: http://kidfriendlydc.com/2011/04/11/sunny-day/. If you don't want to venture too far outside of DC, the National Zoological Park is near by Rock Creek Park and it's a fun activity. Dupont Circle is also a fun place to people watch, picnic, or generally just hang out. It has a fountain and there's enough space for kids to run around. I once babysat two boys (5 and 7) there and they had a blast playing chase and hula hoops, and there are so many restaurants and shops nearby that you won't have to go far to get lunch. Also, I don't know how old your kids are, but if appropriate you can also rent bikes from Capital Bikeshare - just make sure you have or can rent some helmets.

Theodore Roosevelt Island National Memorial is a nice place for easy hiking in the woods with the kids. There are rocks along the shoreline for them to climb upon and my family and I bring lunch and enjoy the afternoon eating by the water. April weather may be a bit chilly here, so jackets and a waterproof blanket for the ground would be ideal. Hope you have a great trip!

Link to the Park site:

http://www.nps.gov/this/planyourvisit/index.htm

Water Mine Family Swimmin' Hole is a Fairfax County Park that has a lake and a water park. The water park is great because it contains a lazy river and multiple watersides for all ages.

We like it most because you can bring your own food and make a day of it relaxing and picnicking in the shade of large umbrellas. It would be a good way to relax when taking a break from the typical sightseeing.

I believe the watermark is only open during the summer months though. Great Falls National Park would be another laid back place to visit.

Good luck! 

Depending on the time of your visit, you may be there for the Cherry Blossoms festival. The National Building Museum is always fun and hands on for the kids. You may want to go to Arlington (Virginia) to watch the changing of the guards and walk up to Lee's House to see the view of DC. Walking around Georgetown is always fun. You may want to head up to the campus of Georgetown University, which is nice. The waterfront in Historic Georgetown or Alexandria (Virginia) are always nice to visit.

National Zoological Park is great for both kids and adults. Lincoln Memorial, Thomas Jefferson Memorial, Washington Monument can't be missed. 

I'd recommend the Rock Creek Park Nature Center and Planetarium located in Rock Creek Park.  In addition to the trails, you can tour the horse stables, get up close and personal and even sign up for lessons.  The planetarium isn't the biggest but it gets the job done.  Perfect for kiddos.  More pics here.