Richmond can easily be done from DC as a day trip - the Confeferacy Museum is not big, but is very well done - the exhibits provide some keen insights.

When I visited about 15 or so years ago I remember being impressed by the "fairness" and objectivity of the depiction of the conflicts, both military and home front. No jingoism at all.

Skip Williamsburg - tourist trap.

You would then have plenty of days for one or more other destinations.