I agree with the Original House of Pancakes, La Madeleine and Tastee Diner recommendations. La Madeleine is a chain, but it's a great place for a good and quick and not too expensive breakfast -- you order food at the counter, your coffee and croissants will be given to you there, but any warm entrees will be brought to you at your table.

There's also a breakfast/lunch place called Old Georgetown Grille on the corner of Old Georgetown Rd and St. Elmo (if memory serves me correctly); I myself have never been there, but I often see people waiting to get a table on Saturday mornings.

If it's chain coffee store you're after for just a cup of joe and a muffin, there's plenty of those in Bethesda -- I can think of three Starbucks in downtown Bethesda, as well as a Caribou Coffee (which I personally prefer) on the corner of Old Georgetown and Woodmont. If you want to pick up some snacks to keep in the hotel room, then there is a Safeway grocery store on Old Georgetown Rd (next to La Madeleine, basically between Woodmont and Wisconsin Ave).

All of these, however, are a fair distance from NIH itself. But if you're at a hotel in the downtown Bethesda area, you're in luck, as Bethesda has dozens and dozens of restaurants.