* Playseum- 7000 Wisconsin Avenue - Lower Level, Bethesda, 9:30 to 6 p.m. Mon-Sat
(full disclosure: this place is run by evangelical Christians; We've never been, but friends love it!)
* The Building Museum, 401 F Street NW, between 4th and 5th Streets, @Judiciary Square Metro (Red Line) 10 a.m. Saturdays; 11 a.m. Sundays
(The Building Zone, for toddlers and pre-schoolers--lots of play space, controlled number of kids/parents at any one time; Also, the museum has a Firehook cafe, and tons of running around space; a fountain)
* Smithsonian Museums, 10 a.m. Saturdays and Sundays
(We have loved The Museum of the American Indian and the Natural History Museum)
More ideas here: A Parent in Silver Spring (blog)
* Cabin John little train
Off Tuckerman lane, pass the entrance to park where DCEG picnic is held, turn left into Cabin John park. The park has a fabulous collection of play-ground areas. It's well shaded for the summer. There are bathrooms, but we've never had to use them. Ample parking. The train is $1.25 for all passengers, regardless of age. You can pay with cash/credit. It's about a 20 minute ride. You can rent a room there for a party, too.
* Glen Echo park & carousel
Less shaded than Cabin John. A little harder to find/get to--off Canal road. They have an okay playground. The carousel is beautiful. There's a little Irish pub nearby that's pretty kid-friendly.
* National Zoo
Open dawn to dusk every day. Lots to do. Parking at the zoo is $20 unless you're a member. Ample neighborhood parking on either side (Adams Morgan or Cleveland Park). If on metro, get off at Cleveland Park and walk down the hill; then to leave walk down hill again to Woodley Park.
* Turtle Park Sprayground 10am-7pm, Mon-Sun
Near American University, off Van Ness. Parking on the street. Shaded playground with big sand box, several climbing areas. Not as many swings as I might like. Some running around space in the playground (a nice hill, etc), but there are also tennis courts and fields that are more open. The splash-park is small-ish and mainly for tots, but a ton of fun. It's not fully shaded. There isn't much seating for parents so be ready to sit on the ground and maybe get wet. There are bathrooms, but I've never used them. Friendly parents: At least once another mom gave me a diaper when I had a swim dipe but no backup for once we were done splashing!!
Glover Park Farmers' Market: Off Wisconsin Avenue, at the school across from Social Safeway. There are usually musicians. No shade whatsoever.
Dupont Farmers' Market: Very crowded, usually there are musicians at either entrance. Not the best for kids.