We live right outside the District, and have a 10 and 11 year old, so we get to do day trips on a fairly regular basis. One of our kids' favorite things to do for a break from the culture is to go to Cosi (sandwiches, coffee, etc) for s'mores. Check out the site getcosi.com to find the one nearest your route and call to make sure they have s'mores. Also, there is a carousel on the Mall, and if you're lucky, you can ride on the dragon. I definitely vote for the zoo, but take a stroller for the 3 year old, unless he is exceptionally tireless. It's HUGE, and on a steep hill. It is free (well, sort of, your tax dollars pay for it).