There are all kinds of uses for old machines, but if you know some kid who'd be able to do his or her homework on it (typing, spreadsheets, presentations) then find them a copy of Clarisworks 4 or 5 and some system disks and turn them loose. They can get a cheapo Imagewriter printer off the web and off they go!!!!
As for OS, 8.1 runs on a Centris and a 475 (depending upon how much RAM you have) but it isn't fun to watch. I loaded it on my Quadra 650 (a much faster chip) last week and it crawled. Due to the lack of memory in the Centris and 475 machines, I'd download (with a more recent machine) floppy images for System 7.5.3 from Apple, and the updates to take it to 7.5.5. That's about all the system that a 36M max (475) and a 72M max (610) should be asked to put up with. 7.6.1 runs well on a Centris, but not so well in small RAM.
Be sure, too, to allocate as much RAM as possible to the applications that you're going to be using. RAM partitions make a big difference on those old machines. Good luck.