You should double-check with your specific car rental company, but you can probably rent a car at National and then return it to Dulles for the same location drop off rate. That might be your best bet on sightseeing on a limited schedule. You could drive down GW Parkway and see the monuments from the river, then cut across the Key Bridge to Georgetown and back track down to Constitution (or Independence) by the museums up to Union Station and Capitol Hill.