DH and I are taking the girls to Washington DC in a few weeks and I'm trying to have a plan because I'm that kinda gal

Babybartfast is 4 and Bittybartfast is still tiny enough not to care what we do. Anyone have suggestions for things Babybartfast will like AND which are accessible by Metro? We'll have a full week to play around. So far my list is:
- zoo
- Smithsonian museums along the mall
- Bureau of Printing & Engraving
- Botanic Garden?
Anything else worth seeing that a 4-year-old would enjoy? I don't think she'd be up for all the civic stuff (monuments, White House, etc.) unless they're free and we can spend all of 10 minutes saying "See that? Cool! On to the next thing!"