"A traditional with an element of humour or surprise" <-- I think this, for that particular challenge / competition.
I say that because I believe you will get a lot more visits, and so more votes, than some other types of caches.
But for myself, my preferences are caches with a bit of a physical challenge, or walks to quiet rarely visited places.
http://www.kammneves.co.uk/bpjc2/I was thinking of having a go myself, but I don't have anything really clever in mind at this moment.