Well, there will be some limits as to what you can do with OS2.1, but I set up my late father a pretty nice looking A600 setup. Things like NewIcons4, NicPrefs, etc.
All other ideas are great too. ALso, with an A600, you prolly won't be able to run WB in a whole lot of colors, so you could use an image processing program to color-reduce background pics, and re-map to your palette. Not sure if it needs OS3.1 or not, but I have BootChange (from Aminet) to randomly select WB and window pics, and system sounds...
