There isn't a ton of great ice cream options as far as DC goes. There are generally a few flavors available at some of the local gelato shops, and if you're lucky, there is soft serve as the DC soul food restaurant Everlasting Life. My main go-to for "ice cream" dates throughout the summer is the non-dairy frozen yogurt carried at a number of shops like FroZenYo. My cup normally consists of a tiny bit of frozen yogurt & tons of fruit & mochi. If you haven't ever had mochi, they are like pillowy sweet little marshmallows. Most people make daifuku, which is filled mochi. But that's not what I want in the cup with the rest of my dessert, so I didn't worry about the extra steps.
Buy a pint of Ciao Bella passion fruit sorbet, a box of mochiko (glutinous rice flour) at your local Asian market, and you probably have everything else you need at home. I used Vegan YumYum's recipe, and it takes around ten minutes start to finish. If you cut the mochi with a pizza cutter, it's even faster. Which I realized half way through my process of cutting tiny half inch squares with a chef's knife. After it cooled totally, I tossed the squares in a mixture of half powdered sugar/half potato starch. Easy peasy last minute end-of-summer treat!