The only drawback to waiting for cherry blosson season is it seems like every middle school in the country is in DC on a bus tour. Was there 4 years ago in April and you are surrounded. Clogs up the attractions.
There's some truth to this, though any attraction worth seeing won't be clogged
Definitely skip the tour of Congress, it is a joke, you see nothing, nor can you, unless you are buds with your state representatives, and it is too crowded at any time of year, and the security is ridiculous.
You'll see the visitor center, the old Senate chamber, the "vault", and about a quarter of the rotunda, and that is IT. If you're tall (and you will be with all the asian tourists), you'll be stashed in the back. Oh, and if your state's statue isn't on the main concourse of the tour, don't go looking for it, even if it is just 5 feet off of the concourse (as MD's was) because security doesn't take kindly to that.
Make sure you do NOT go into the side of the National Archives which faces the mall. You actually can't see any records if you go in that door, and the viewing room for our historic documents is tiny, dimly lit, and crowded. You get a really kiddified museum exhibit on that side--National Treasure lied. Go in the other side of the building.
The Library of Congress is definitely worth it--getting a card is ridiculously easy, and the underground tunnel complex is awesome.
TR island is never packed, for example, but the FDR memorial always is (and that's at the cherry tree ringed tidal basin)...and you'll see all the little kids who pull his finger and ride his dog, which keeps them gleaming while the rest of the statue is oxidized.
Post Office Tower was awesome, though not if you're claustrophobic.
Definitely use the metro. Driving in DC Land is not worth it, though driving in the immediate vicinity isn't too bad depending on time of day.
While on the Metro--which is only 7.80/person for a day pass--check out Union Station. While AMTRAK is something our country should be ashamed of, the architecture is awesome, and the Columbus memorial and the Jap WW2 memorial are right there if you're into that.
(Unless you have to travel before 9:30 am, the day pass is the better deal, especially if you park at a terminus of a metro line. To go from an outer stop into the city and back costs more than the pass)
Oh what else, what else...there's way too much to do in DC land.
Lincoln is easy to see, it's never too crowded. Jefferson can be a bit packed, but nothing major.
Washington Monument is something you have to get there ultra early for. You can only go up if you have free tickets, which generally sell out before noon. You can't have someone else buy tickets for you, and you can't buy tickets for another day. Oh, and you can't use the stairs at all, so if you wanted to see the historic stairs, you're SOL.
(TR Island is still a cool little side trip if you're bored)
The cruises on the Potomac to Mount Vernon were cool, though I don't know if they run now, or if you're interested.
I generally just drive to a metro station and pay the 4 bucks parking fee, or park for free on weekends and holidays, and if you should park there and pay, remember that only certain stations accept credit cards, and none accept cash. The others mandate you have a plastic SmartTrip card, which costs a minimum of 10 bucks to buy
Oh, and most importantly, carry no metallic items. It's not worth explaining to grunt security dudes every time that having more than 3 keys on a key ring does not make it a "weapon", and having to take off layers of clothes just for a belt buckle or a shoe fastener is ridiculous--and have happened either to me or to people I've gone with repeatedly. Security at Congress is again ridiculous--you'll have to throw away ALL food and water, and liquids, before entering. They provide trash cans for this, and they DO search.
...and Right Said Fred's I'm too Sexy should be playing as they seem to enjoy making people take off their clothes en masse
(Which is another reason to avoid Congress)
Have Fun.
