I'd put Linux or Unix on them and play free games like Wolf:ET.
Linux because - was shopping for Windows on Ebay and found Ubuntu 6.04 cds.
Had to learn basic bash. Coding started from there.
Internet wasn't the best, so I setup my own squid proxy, then web servers, etc. By HS I was writing code for fun and profit.
All thanks to some old Pentiums. :)