The Smithsonian museums are walk in only. They are free.

The Washington Monument, the Spy Museum and maybe the Newseum have options to get tickets ahead. The Holocaust Museum has an option to get a ticket ahead of time certain times of the year. They are timed tickets but the Holocaust may be anytime of after the stated time.

Spy and Newseum are not free museums. Holocaust and Washington Monument are free but if you get a timed ticket you have to pay a service charge

You can get a timed entrance for the National Archives. That is completely free no service charge and no entry fee.

US Capitol, White House (need months in advance), Holocaust museum and Washington monument are available and timed. Since the Smithsonians are free, there are no advanced tickets. I never saw a line for them either when I was there in June.

Here is my trip report.
http://www.fodors.com/community/united-states/trip-report-washington-dc-for-red-sox-games.cfm

Our favorite museum was the Portrait Gallery (wasn't even on our list of must-dos). I loved the Air & Space as well. Given 3 days, I would definitely plan ahead and pick & choose what you see in each museum, rather than trying to see all of them.

Another site which requires an advance ticket is Ford's Theatre. The tickets are free but if you order them in advance, there is a service fee charged.

The only time Smithsonian museums have tickets is when there's a BIG exhibit on; otherwise, indeed, they're free and you can come and go as you please. Ain't that nice?

My advice as to what to see in 3 days is this: sit down with a guide book and make a list of your must-see's. There is so much to see, so many monuments, museums, historic homes and government buildings that it would take years to see them all. So most people do the major monuments (Washington, Lincoln, Jefferson, etc.) either on a night tour, or on foot or by bus during one day (it's a LOT of walking); then another day is devoted to museums, of which you should choose 2-3; and a third day to National Archives, Congressional Tour, White House tour (if you can score that), Library of Congress (not all of these, just the ones of particular interest).

Mind you, that's a very rough guide, and just my own approach. But I do think you have to see the major monuments (Washington, Lincoln, Jefferson, Vietnam War memorial), Capitol Hill and the White House, even if only from the outside; the American History museum and the Air and Space Museum (though my fave is National Gallery of Art, btw). The rest must be dictated by your personal interests...

While we're on this topic, any museums to avoid on the weekend, assuming it's more crowded on then? We're going the first week of April during the Cherry Blossom Festival.

Some museums we're interested in:
American Indian museum
National Gallery of Art
Hirshorn
National Cathedral

BTW, how to reserve tickets for the Capitol tour?

Thanks!
Jen

Overall everywhere will be busy during the Cherry Blossoms but the AIM is not the most heavily visited of the Smithsonians so you may find more breathing room there.

Capitol tour = through your Senator or Rep.

I know you know this, but the National Cathedral isn't a museum! VIsiting hours on Sunday are limited b/c of services. The NC does have a requested donation for visiting.

Just back ....National Gallery of Art was crowded both Sat and Sunday. Crowds seem to be highest about 4 so we went early and went back to hotel then.

We saw pandas early morning.....no crowd.