On that note...

What about Philly? Take the train (or drive). They have great art museums plus I heard the Franklin Institute is great. Great food in Philly You could make an afternoon of wandering through South Philly, eating, if s/he is adventurous.

If it's just a 3-4 day weekend I'd consider whether you, and especially your 9 yo, want to spend time at the airport and flying. We tried this once (3-day weekend trip to Chicago) and our departing flight was delayed 4 hours, and our return flight was delayed 2 hours last minute. Adding in the advance time required to be at the airport, it was a real lesson for us.

But it certainly can be done.

Hershey Park
Busch Gardens & Williamsburg

I visited St Louis recently on a work trip. It was depressing. I would not do a family trip there.

Charleston, Asheville, Boston, Chicago? Yes. St Louis? No.

Colonial Williamsburg
Boston
Virginia Beach/Norfolk
Assateague or Chincoteague

Thanks for all of these ideas, especially the ones in the Midwest. They aren’t things I would have come up with on my own!

If you’re willing to fly, Montreal, Boston or Chicago could be fun and special.

Great Wolf Lodge?
Williamsburg?

Is your nine year old a 4th grader? Ideas and special perks here: https://www.everykidoutdoors.gov/index.htm

Indianapolis, home of NCAA and one of the best children's museums which happens to include huge sports area
https://ncaahallofchampions.org/visiting-information/
https://www.childrensmuseum.org/exhibits

St. Louis, for a combination of history/national park experience at the Arch and the City Museum which is unique and super fun
https://citymuseum.org/

Blue Ridge Parkway/Asheville - hiking, biking, ziplining and the Biltmore, arts districts

Williamsburg, Jamestown, Yorktown, and Busch Gardens

Chincoteague and Wallops Island

Are you off-season beach people? Ocean City and Assateague, DE beaches, or Virginia Beach/Hampton/Norfolk

Where are you located? If you are in DMV I'd recommend a road trip to the triangle area in NC, if you haven't been there before. It's a 4.5hr drive.

One parent is taking one sibling on a ski trip out west. The doesn’t love to ski 9 year old is staying home with the other parent but has a long weekend that skiing siblibg doesn’t have in May. we’d like one parent to take the 9 year old someplace fun but she doesn’t have any ideas of what she’d like to do. NY is not a draw as we go there frequently to see family. She’s into basketball but unfortunately she’s off the weekend before the WNBA season starts. Ideas of a fun long weekend for a 9 year old? Driving or short flight is fine.