You can get a library card for all DC-adjacentish counties (and the Falls Church Mary Riley Styles Library) with an ID card and a piece of mail with your address. I think there are 8 reciprocal counties? Gives you very different audiobook libraries for Libby. Free storytimes and activities at each branch too. Good for entertaining babies.

The best parks I have ever seen. Borrow some kids if you don't have your own so you have an excuse to try some of these climbing contraptions.

The CAPP is reasonably decent for licensing paperwork so you don't need to go in. (Pet, rental, business license, etc) They are really responsive about merging/updating accounts by email (think minutes during business hours).

Free mulch. You haul or pay a small delivery fee to have it dumped in front of your house.

There's a yearly open house at the city works yard near Shirlington so you can sit in an excavator, garbage truck, etc.

The sketchy BBQ cart by Shirlington is pretty decent.

Alcoholic drinks to-go in Shirlington businesses.

Thought of another one:

Long Branch Nature Center (and the other nature centers) has an indoor reptile and amphibian zoo and a playroom with dress-up and activities. It's free.