There's a canal just south of M St. (the main street in Georgetown). Walk down to it. Specifically, walk down to it right by the Barnes and Noble store. You'll see a coffeeshop just past the canal, go in! It's nice. Take a walk down the canal and then eventually you'll get to some steps that will take you back up and out by the Dean and Deluca. Also a good coffeeshop there and you can sit out on the pavement there.
The basement of Pizza Paradiso has a spectacular hidden bar.
There's a French restaurant on M St. with a great "summer salad".
Enjoy!
Without knowing much about what you're looking for:
Try to make sure you go to Adams Morgan (specifically around 18th and Columbia, about a 10 minute walk or 2 minute Circulator ride from the Metro).
Dupont Circle is fun too. The monuments are a definite win, of course.
Shameless plug, check out my startup's (relevant) property here: http://dateideas.in/dc
http://www.bohemiancaverns.com/history.htm
Also recommended, visit Newseum if you get a chance, as well as walk around Georgetown.