A Food Truck Tasting? I would contact the trucks ahead of time and see if they’d give you a group discount (and maybe smaller portions for tasting).
Otherwise, here are some other ideas:
Rent out Penn Social during the day – Lots of fun games
Private Cooking class:
L’academie de Cuisine – http://www.lacademie.com/
Culinaerie – https://www.culinaerie.com/rescalendar.php?view=3&publicview=1
Segway Tour of DC
Movie Marathon – Multiple Movies in one day or Rent out a Screen for a few hours