I disagree slightly. My 3 year old son loves riding the metro more than life itself so I consider myself a professional metro rider.

DCA is on the blue line and you can make things simple to avoid the chaos of navigating the metro:
- Ride blue line toward "Largo town center" (get on the train on the airport side of platform)
- Ride it up 11 stops and get off at Smithsonian (~ 20-25 minutes)
- Quick walk down past Washington Monument around the water in front of Lincoln and back (~30-40 minutes)
- Get back on blue line toward "Springfield Franconia" and ride back (~ 20-25 minutes)
It's tight but doable. If delays result in your free time being less than two hours, don't do it.