“What makes an instrument fun to play?”
I would say that any instrument that succeeds in mediating some sort of artistic communication from a performer is one that is fun to play. This would include everything from a violin to an Ondes Martenot to pretty much anything else theoretically.
What makes an instrument fun to experience as an audience member?
I would say that any instrument that is either traditionally aesthetically pleasing (one might use the words “harmonious” or “consonant”) and/OR one that is interesting/unusual would be fun to experience as an audience member. I think the Ondes Martenot is interesting mixture of both whereas the guy with the recording gloves in the first video may be more of the latter.