Affiliate freerolls are kinda dumb IMO and probably provide you with close to a $0 ROI.
Something that would be more exciting (to me at least) are some kind of vacation. 1-2 weeks somewhere really nice with internet access so that we could still work on our sites, but from the comfort of a nice place somewhere warm (preferably not during hurricane season).
Another idea is tickets to an event that is extremely hard to get tickets for.
Cake did a good job with the Playboy Mansion thing, but I didn't like how you had to play out a poker game to actually win it.
I think a good way to structure something like this would be to say that the prize goes to the affiliate who brings the most players that rake over $50 or something in a 3 month period.
Also, if there are 2nd and 3rd place prizes more people will be encouraged to give it a shot.
Those are my thoughts on this... good luck!