Other places to watch would be some of the parks along the Potomac. A popular one is the one west of DCA just off airport property. You can metro to the airport and walk to it or if you are driving you could probably park at the airport. If you actually want to park in the lot for the park the entrance is off the GW parkway, but you will need to get there early.

I used to live in the DC area and getting downtown for the 4th is chaotic. Any public place that has a view of the fireworks is going to be packed. I was lucky enough to have friends that lived or worked in buildings that had good views, but getting home was never fun after those.