If it were me and I wanted to have a fun "non-boring" time for 3-4 days, I'd plan...
A Capitol Tour (Capitol Visitor Center) - preschedule online
A trip to 2 favorite Smithsonians (depending on kid interest - I would go for American History, Natural History, and Air and Space as my top 3, but if you have art buffs, go there - the art museums also are the least "dumbed down" and stay at teen age level and above

- done on a whim whenever
The International Spy Museum (adding on Spy in the City or Operation Spy) and/or the Newseum and/or Madame Tussauds (whichever is of greater interest to your kids - get a deal on tickets - they are always around) - all these museums cater well to this age group instead of "dumbing down" all of their exhibits - again, do these on a whim - if you have to pick one, pick either the spy or newseum
The Cherry Blossoms - I might rent a paddle boat
http://www.tidalbasinpaddleboats.com/ or just take a walk on the Tidal Basin (seeing the Jefferson Memorial while there) and I'd check what events I could see
Some fun - the area trampoline parks or escape rooms could be awesome to add in
I'd do much more if it were a week (seeing the Archives, the Monuments, Mount Vernon, Arlington National Cemetary, the Supreme Court, BEP, Holocaust Museum, etc) and my kids were totally into history...but you can overload on history and museums that are "museumy" - what I suggest above gives you a bit of everything to try and gets your appetite whet to come back to see the rest on a future visit

...most of the museums should each take a 1/2 day if you're gonna really explore them and you should enjoy each one as they are rather than rush, rushing through everything

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