Depends what do you want to do but I'd say Dell & Lenovo micro PCs. x86, changeable and upgradable CPU, RAM, and storage. And they are stil very very small https://www.youtube.com/watch?v=K-x0mVgDABA I have severall Optiplex micros at home for selfhosting, playing around VMs etc and I love them.

In Europe you can get one with a 6th gen i5 CPU, 4GB RAM and SSD for ~100€. And of course you can upgrade the CPU for something better, add 32 GB RAM, bigger storage etc.

No GPIO pins though.