Make it into a musical instrument/recording and mixing machine. I still support those directly (airwindows.com). I also produce VST2.4s that ought to work for

Make it into a musical instrument/recording and mixing machine. I still support those directly (airwindows.com). I also produce VST2.4s that ought to work for your Linux versions of those same Macs. Fairly often they have useful things like digital optical ins and outs. My primary development is on an old laptop running Snow Leopard for just this sort of reason. I'm pretty sure you can get versions of Reaper (or the current version) for machines this old. If you want to get fancy put a SSD in the old machine, which will get a lot more out of it.