Give me some suggestions 
In no particular order:
+1 for mastering the CLI. Always good to know your way around the shell.
Learn ARexx and then how to use it to do funky stuff with your existing apps that support it.
Give OctaMED Soundstudio a whizz.
Get the NDK and have a go at C.
Try your hand at a spot of 680x0 assembler. Compared to many other architectures, it's fairly easy to get to grips with and nice to write, to boot!
*Create a crazy boot scheme where you create a RAD on cold boot and extract an archived minimal OS installation into it when you boot, using RequestFile to select the "boot image" from a directory.
*I actually do this on my main A1200T.
