Amtrak gift card if she has interest in visiting other east coast cities.
My sister gives my niece a pass for those red bikes. Niece walks to work and jogs around town a lot, but the pass makes it easy to just jump on a bike whenever she needs to get somewhere fast. Niece doesn’t have a car but she has friends who do, and her brother just moved to Reston, so her shopping life just got easier.
Good idea @Embracethemess . And yes, @doschicos - she has been and loves Philly. I sometimes wish we lived out East where there were trains to spirit us away to new towns to explore. Sigh, but we do have very affordable housing 
@Onward we live in Philly and my D just moved to DC for her first post-grad job. She will be getting Amtrak gift card for Christmas so she can splurge for the train versus the bus when she wants to come home for a visit! (Or to go to NYC to visit her friends living there.)
I work in downtown DC and a great place (if you end up needing any other ideas) is Artechouse with these great interactive light and tech shows/experiences. The one opening now explores the Northern Lights. https://www.dc.artechouse.com/
I did a cooking class at Sur La Table with my D and my sister. It was an xmas gift for D and I from H. It was really fun, but not cheap. Definitely worth the price and you do get a discount in store on the day of the class.
@Barbalot - that looks cool and so inexpensive!
If she enjoys a tipple - How about a mixology class - DC is choc full of great cocktail bars!
A butchering class (I’d love that!)
A romantic dinner for two - at home, w private chef!
A beauty treatment she may not gift herself i.e. a few months of manicures
Walking tour of DC
Gift certs to Restaurant Week (I think its in April) a restaurant a day for a week / weekend - great deals to be had including some Michelin *
Giftcards to Trader Joe’s if there is one close to her home and if she is a fan.