Lots of good suggestions. For anyone not just OP, If you are going to stay outside of DC besides Crystal City/Pentagon area. I would suggest in looking at Old Town Alexandria area.
You can take Metro from King St Station to DC.
Hotels with-in Walking distance on King St (this runs to the river approx. 1.2 mile long and has free bus that runs the length every 20 minutes or so). Lots of restaurants, wine bars, etc.}
- Hilton Inn
- Hampton Inn
- Lorien Hotel and Spa
There are other hotels on King St further down but these 3 are the closest to Metro.
near the water there are water taxis to National Harbor (low priority for a short trip- newer with hotels and restaurants but expensive)
Old town Alexandria also has Ghost Tours in the evening and a few ice cream/candy shops if you need to add some weight back on from all the walking.
Definitely in DC stick with the Natl museums and monuments primarily. Although Ford's Theater I think is worth the fee due to the history. One thing that you can not plan on completely but look at calendar as far as when Congress needs budgets passed or when Govt runs out of money. If Govt shuts down all Museums are closed, even the monuments. (Not trying to get political). It just that the govt shutdowns in the past have caused havoc to vacations for people.