One cool place that I don't think people know about/ take advantage of is the National Arboretum. That would depend on the timing of your visit though.
The big secret about D.C.--Baltimore is a lot more fun. You can drive to Baltimore watch an Orioles game, eat crabcakes in the ballpark, go to the nearby inner harbor, take a harbor cruise, get a water taxi pass, and get to most everything worthwhile. They've got an aquarium. They've got ships to tour. They've got bars and clubs with character. They've got historic fortresses. Just pay your money and park someplace decent and safe because you don't EVER want to get lost in downtown Baltimore. Hence, the water taxis.
If you're wanting to see a lot of the major sites in DC in a short amount of time, I'd recommend one of the open-top tour buses... We lived in DC for couple years and did the bus tour a few times with family/friends that were visiting and only had a short time to fit in as much as possible.
Figure out which museums interest you, choose what you really want to see as you can't see them all, American History, Natural History, Air and Space are my favorites. Do the mall and all the monuments of course, if you like agood nightlife, you can always go out in Dupont, Chinatown, adams morgan...
If you want a tour of the Capital get it through your reps, Senators are better than Congressman, the public tours are lame.
I definitely agree that the National Mall is AWESOME, but if you're coming with kids it all depends on their interests/ages. I always loved all that stuff when I was a kid, but I've seen a ton of kids bored outta their minds during trips with schools and family.
Old Town Alexandria is really cool. There's plenty of shopping and good restaurants.
Hence, the water taxis. 
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