Oh, the most socially mundane things you can imagine that would be hilarious for a prolific killer to do. This bitch (gender neutral) is really struggling;

Fabric crafts. Knitting, cross-stitch, crocheting, you name it.

Model building. Trains, animals, buildings, dioramas.

Bird watching. You join an online group and everything.

Collecting items. Coins, postcards, fucking bottle caps. You don't travel as much anymore, but that's what online auctions are for.

Plant rearing. I'm talking bonsai, marimo/algae/, orchids.

Reading. This one is probably the least socially boring, but you've been reading nothing but children's books for the past several years- you're finally branching out!

And more, if I can think of them or get fun suggestions!