You can lead them to a CPA site to fill a survey or any other content locker you like and get a few bucks. But a list and new followers will be more valuable and you will be able to monetize it more than once with different offers, you may even sell it for more money than you would get with surveys. People tend to give up when they see a survey, but they are more willing to give you their mail for a contest.
If you give a prize for real, better for you, make sure to show the winner with the prize to all the contest participants, so they will share more easily your future contests.