"when we aren't traveling"...if she gets to travel every summer, you're already doing more than many parents of teens outside of dcum bubble.

My oldest is a pre teen, but here's what we do, though we live in CA so we have weather on our side pretty much all summer.
-state parks pass + regional pass for parking year round at rivers, beaches etc. makes us go more knowing we don't have to pay the $10
-paddle boarding
-swimming in our apartment pool + grilling, or grill at the BBQs at the parks
-drive to bay area for the museums/aquariums
-sporting events, usually baseball
-water parks, usually the city pools w/slides unless I want to splurge on the big ones, pack lots of food
-walk to get ice cream or Jamba at the outdoor mall down the street
-Drive in movies

We go out for ice cream every Monday night, sometimes the regular spots and sometimes we try something new. Not sure that is spontaneous but it feels that way.

Get a poster board, cut it into smaller pieces for everyone in the family and have them write what they want to do this summer.

Urban hiking or biking, like from the National Mall to the Cathedral or on the Anacostia River or Capital Crescent trails.

Visit a pick-your-own fruit stand, then make a special dessert with the proceeds. (My teens are still very much into this.)

Geocaching is a fun way to explore new places.

Drive-in movie, planetarium, outdoor ropes/ninja course, boat rides, local gem mines or gold panning, outdoor sketching excursions if they like to paint or draw.

Busch Gardens and spend the night close by

Virginia Beach (or some coastal location) for a night or two

Shopping spree at the mall - give them some cash and let them figure out how to spend it

Spa day for the two of you

We have blow up paddleboards so will go out and play on the Potomac or lakes around. Will decided to go to a Nats game the day of if teh weather is nice and we like who is pitching. Concerts. Urban hikes out for dinner. We live in DC. We particularly like to take them to some of our old favs like Zorbas. Camping. Rafting/tubing. Sandy Springs. Used to randomly do Six Flags.

My son and I found an out-of-the way ice cream shop that we enjoy. Close enough that we can go spontaneously, far enough that it feels like a deliberate treat.

Lots of good ideas already.

Others…
Massages, mani/pedi together.
A new restaurant
Movie theater
Live show
A festival - on the mall or July 4
Wolf trap show
Lots of live music around in the summers
Skip stones
A volunteer event
Escape room
Project type place- woodworking, craft, fiber, sewing, painting, ceramic etc
Family paint night
Game night
Hershey park
Baltimore harbor
Harriet Tubman Underground Railroad expedience trail
Swimming
Rock climbing
Farm to pick fruit/vegetables

We still make a summer fun list (I have a teen and a tween). We don’t do everything on it, but it gives us ideas and fun goals. Things on it usually things like:
- swim & hike at Cunningham Falls State Park
-fossil hunting at purse state park or flag ponds
-zoo
-outdoor live music
-aquarium in Baltimore
-metro downtown & see a couple of museums
-Harper’s Ferry
-Annapolis
-explore a new-to-us park
-Billy Goat Trail
-Flying Circus

Further afield:
-Shenandoah
-Luray Caverns
-Philly
-Historic St Mary’s City
-Shenandoah River State Park