My son isn't quite as interested in alleys as I am, so I got my husband to come with me on this one. We started at Pomander Walk (south off Volta between 33rd and 34th), then went to Cashell Alley (which was just named and doesn't really have older buildings or homes on it so you could skip it, but it's right there north off Volta between Wisconsin and 33rd). Then we went up Wisconsin to Caton/Scott Place (Reservoir and 32nd- you can walk through it on Google street view) and over to Orchard Alley (NW of P and 30th), then down to Corcoran and Oak Alleys (between 31st and Wisconsin and M and N). We crossed the canal to go to Cady Alley and Cecil Place, then into Georgetown University and back to the car.
I loved it.
If you think alleys are interesting, you might like this pdf about alleys in DC.