You're in DC??? Oh, I'm so jealous! I lived there one summer for an internship, and I fell in LOVE with that city! It's my dream to live there some day.
Just a word of advice: when you go down to watch the fireworks, get there EARLY! A bunch of my friends and I wanted first of pick of a spot, so we camped out there the whole day. Probably not 100% necessary, but get there at least a few hours before dusk. And be prepared to wait in a long security line just to get into the fireworks watching area. I'd suggesting sitting down around the Washington Monument rather than on the Mall, there's a better view from there.
P.S. I don't know where you're staying, but there's are grocery stores around, they're just hidden.

I can't remember where they all are, but I know there's a Safeway in the Watergate building, that's where I usually shopped. Have fun!!!