Lots of people saying take the metro on here, and it could be good, but if you don't live near a metro station or have to tote around kids, bags, other stuff, that would be rough. But yes, if it's just you (and maybe another adult) with a single bag (each), metro is likely the best option if you live reasonably close to it.

If you live close, might be good to taxi/lyft/uber as well. Or if the above would suit you minus distance to a station, maybe taxi/lyft/uber to a station and go from there.

Unfortunately DCA doesn't seem to have any good offsite parking outside some hotels that might offer it.

It's really hard to know what's good for you since we don't know who/what you're traveling with and where you live.