The role (and the company) is already remote (my strong preference) with "unlimited" PTO, and I'm short on creative ideas!
I saw a colleague who specifically put in his contract that he always had whatever dates this one convention occurred in off.
Before I was a remote worker, I had it put into my contract that must be allowed to leave work by 1 hour before sunset. I have poor eye sight at night, and must drive when the sun is up for safety. I never wanted to be penalized for taking off early in the winter. Someone could say I could invoke ADA, but it's just easier to just have it in the contract up front.
I had a colleague who negotiated an specific office chair because he has a bad back. It wasn't super expensive or anything, but important to him.
That's all I could think of off the top of my head.
An office with a door.