Soft stuff is good practice, imo. I'll try out new tooling or setups with PVC. Especially threading. If you can run low speeds, you can thread PVC pretty nicely. I would grab a piece of plumbing scrap and turn and thread it, just to see how it comes out. Delrin is very nice to work with, so there's pump handles, bolts, pull pins and poppet seats. Very hard woods are good, but you have to go slow like they are metal because they can still crack or strip out, depending on the grain.
You can polish round stuff easily. Some people keep a little lathe just for that purpose.
Another fun thing is coming across a screw fit that just doesn't quite work with the standard head. You shave a little off the side and/or off the top, and it's magic. You could have dremeled it, but that's not the point.