Drive into Old Towne Alexandria, Virginia. Have dinner on the water. Walk around and look at all the cute little shops.

I really enjoyed both the food and the beer at Capitol City Brewery.

See if the Library of Congress has any free concerts or programs in the evening. They offer some really unusual programs.

Go see the monuments at night. I thought this sounded weird when it was suggested to my traveling party, but I found it was all at once romantic, odd, sublime, sobering, and patriotic-chill-inspiring. I will forever remember climbing up these out-of-all-human proportion steps to stand at Lincoln's knees, just a couple of years ago, and read the Gettysburg address in the dark yellow light like I had never read it before. In this time of stress, division, and war, it can be helpful to read:

"Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure.... It is for us the living...to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us -- that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion -- that we highly resolve that these dead shall not have died in vain -- that this nation, under God, shall have a new birth of freedom -- and that government of the people, by the people, for the people, shall not perish from the earth."

It's that kind of stuff that makes me redouble my efforts not to let the preznit have the last word.

Consult old AskMes; this question has come up a couple times and I've used the threads when visiting there.

Oh, and steer way clear of those LaRouchePAC kids! That is some serious Kool-Aid they drink.

The Zoo Bar has awesome blues.

window shopping or people watching in dupont circle. plenty of excellent restaurants in dupont circle too.

Go north along the Canal in Maryland to Carderock Springs, where there is a neat trail along the edge of a gorge in the Patomac. You should still have enough light to hike and the trail is very nice.

Or go farther north, on either side of the river, to Great Falls.

If it is open late enough, stop by the National Botanical Garden (right on Capitol Hill).

Get Burmese food at the Burma Restaurant:

Burma Restaurant
740 6th Street, N.W.,2nd floor
202-638-1280

"Over the years, the service at Burma Restaurant has smoothed and the tables have taken on white cloths, while the cooking has remained steadily delicious. Light curries and stir-fries flaunt fragrant seasonings, and everything from rice noodles to kebabs is mellowed by a garnish of dark fried onions. But the highlights are the first courses: puffed fritters of squash or eggplant, salads of young ginger or tea leaves, and soups that mingle tang and tartness with chile heat, coconut creaminess, and those sweet onions." -- epicurious

And don't miss their green tea leaf salad.

Alexandria's pretty cool, and if you're there during the day, check out some arty shit at the Torpedo Factory. But in DC proper, I recommend the National Zoo, the Building Museum gift shop (right near the Burma), and the new African Art museum: Disney just donated a massive collection of significant and beautiful works of art to the collection, and it's terrific. Is the Folk Life Festival still going on the Mall? I think so...

The National Arboretum is pretty cool, particularly the Bonsai collection, and it might be a good time of year to check out the Aquatic Gardens in Southeast.

Miko's got the right idea about the monuments. The Hirshhorn gallery's outdoor sculpture garden is worth a visit.

Dupont Circle is pretty cool, but Adams Morgan is cooler. Particularly if you like empanadas -- Julia's Empanadas kicks the proverbial you-know-what (ass). And there are great Ethiopian places, particularly Meskerem.

I just put links to everything in here, but I got booted from the edit page as I hit post and lost it all so I feel kinda robbed and unwilling to do it over. But these things are all easy online searches, except for the Burma, which is why I included the review. Sorry for my unwillingness to do it all over again, but those are the breaks.

Have a great time.

Oops! The aquatic gardens are by Anacostia, technically NE, not SE. But don't stray too far into the neighborhood; it's no joke.

Crap, I just read the after 5 part. Still, some of those options might be open later.

There is always the Go See The Exorcist Steps thing in Georgetown. . .

I'm such a homebody loser . . . I have nothing to add.

Hugh: your answer fucking rocked. Thank you!

(Many other answers also rocked, but Hugh earned massive kudos.)