Key Bridge Marriott is a good location. Custis Trail, Mount Vernon Trail and Key Bridge all come together there. You can watch the fireworks from the bridge or go down to the Mall.
Here's a route that gives you a good view, especially at sunrise.
http://www.gmap-pedometer.com/?r=5388052
Lots of choices for food.
When I'm on the Mall, I like to go to the food court at the Post Office Pavilion. Many choices, but the Indian place is particularly good. Take the elevator up the tower for the best views after the Washington Monument.
Eastern Market is also good. There is an eatery inside, but the crepe stand outside is very good.
Georgetown has many choices, some pricey but many reasonable. Tu oTu does good paninnis (I'm a bit biased since I helped them open their second location at the end of Key Bridge, but they are really good.)
Not sure about Rosslyn itself. Many places are geared for lunch. If you go down Wilson you'll find a lot of choices at Courthouse (where I used to live), Clarendon or Ballston. I like Toscana Grill or Summers near Courthouse or Delhi Club at Clarendon, but I don't have a particular strong recommendation.