Where to pop a nice bottle of champagne?
Not a rooftop but the first time I ever had Dom Perginon was when my father brought a bottle to Equinox for a celebratory dinner (my Grandpa's 80th birthday). The champagne was paired perfectly with Chef Gray's Mid-Atlantic Regional cuisine. The atmosphere, food, service (and of course wine) was a dream. I highly recommend bringing your bottle there. You can probably even call ahead and see if the Chef will design a special menu for you around the champagne.
Afterwards take a stroll by the lit up White House.