This has become one of my favorite spots in the whole city. The Park Hyatt Washington is in the West End, on 24th St. NW and M St. NW. An easy walk from the Foggy Bottom Metro. On Saturday and Sundays, the Tea Cellar offers afternoon tea with a $13 dessert buffet (pot of tea is separate).
What makes the Tea Cellar so special is that it offers a tea sommelier. Yes, you can even do tea tastings. The staff there are incredibly knowledgeable and will happily share as much as you want to hear about the various tea offerings (black, white, green, oolong, performance, etc.) Pots of tea range from $5 all the way to $300.
I think the space and presentation are lovely. It's very different from most hotel afternoon teas which I feel can be rather stuffy. The Tea Cellar is a very modern space, all light wood and glass, with soaring ceilings. I love sitting in one of the comfy chairs or couches near the window and people watching. Despite it's large size, I think the space is sectioned off nicely so it can be cozy or open as you prefer. I've been there with just one other friend, but I also had a birthday party of 15 there as well, and both worked equally well.
The dessert buffet has an edited assortment of offerings--alas, no petit fours or devonshire cream here--but they do offer little raisin scones with butter, chocolate torts, madeleine cookies, poppyseed cake, miniature chocolate chip and sugar cookies.