Maybe you could do a puzzle, where clues in one thread lead to another thread, and so on, until you have about twenty threads in a line of which the final thread would give you the answer to post in the competition threads.
You could edit answers into old theads, so that they don't just show up as new posts, and use alternate user accounts to hide clues in Technical requests.
Example of clues;
- Take the number of presidents before Bill Clinton + number of forums in Daniweb x 6 = answer to add to url, (doesn't work btw), or other clues requiring web research.
http://www.daniweb.com/techtalkforums/threadInsertanswerhere.html
- Find a blog about ..... and follow the link
- Find a code snippet about.....
Ok, so I am rubbish at making them up, but I remember Tgreer said that he was into that stuff, maybe he could do it, and the bonus is, you could use it as a way for people to explore the forum more. Either offer a prize for finishing the whole thing, of a small prize for first person to answer each clue, (people can win only once of course).
I think the poll would work well, but only amongst the most popular members, and they would be the most likely to actually vote. It would certainly be a popularity contest, but I wouldn't worry about that. The most sucessful posts, between ocasional users, seems to be ones that share interests, like what music you like, or what films, Some sort of photo competition might be fun, but just choose a winner yourself, and use one post, maybe with an IT theme.