There are a couple of different challenges I've read about before. One is the challenge to do every ride in all 4 parks in one day, they have a strict definition of "rides", and there are currently 46.
http://www.parkeology.com/wdw46-faq
A few years ago, I read a different first-person journal account of a group that was trying to do as many "attractions" as possible at MK in one day. For them, every character M&G, every Main St vehicle, every Castle show or parade was an "attraction", I want to say they got over 50? But I can't find anything about it on the internets.
Craziest thing I've done is hit multiple headliners at all four parks in one day.