Roosevelt Island is nice, with a short sunny section on the boardwalk.  From there you can lengthen your run (if it's not closed after a flood) along Potomac Heritage Trail and head northwest to Potomac Overlook Park - roughly 3.5 to 4 miles one way. Portions run parallel to GW Parkway in the sun, though.  But Potomac Overlook is another nice.  Or Turkey Run or Scott's Run if you like steep inclines here and there!