A common BASIC program I'd write on new-to-me computers was a FOR/NEXT loop to print all the characters in the character set. I remember discovering the color semigraphics characters in the CoCo character set and being weirded-out. I'd never seen anything like that on any of the other machines I'd ever used.
I snared a CoCo2 a few years ago so I could re-live a few of those memories (and explore the machine with actual documentation).
There are several other good sources out there too but it's a good start for anyone looking to relive the fun old days.