I defer to others on the trip (though I would take a cab/car if it were me) but you can watch the fireworks from almost anyplace in the District area if you can see the Washington Monument. We watch from the roof of my apartment building well up the hill from the Mall and you get a pretty good view, not just of DC fireworks but also of fireworks in nearby communities. So ask if your hotel has a roof deck or if they can recommend some other place to go.