I'm not sure about finding a computer that would both meet your idea of a fun machine and meet the aesthetics that they might be used to of modern computer systems.

Have you thought about introducing them to something where they could code/create online and could share their work? You might want to take a look at the online free Logo system at:

http://www.logointerpreter.com/

They would be able to click on the works others have published there and they could tweak the code to do other things.