You probably already know this, but for the benefit of others who may be reading, the first step is to figure out the demographics of the people who liked the page. Once you have your demographics nailed down, then you can go about figuring out how to best monetize it because that really can make all the difference. For instance, you wouldn't want to send Tiffany&Co traffic to "Win an Xbox" offers. On a somewhat related note, it also helps if it's a transcriptional interest, meaning that 100K likes on a fishing page could easily be worth a lot more than 350K likes on a "Free joke of the day" page.
Once you have your demographics down, it's time to do some brainstorming and figure out what things you want to try. Once you've narrowed it down by interest (or likely interest), use some of the suggestions here in this thread and make a shortlist of ways to monetize, pick out the best ones, then start testing as Denix3 suggested.
I hope this helps a bit, and I wish you the best of luck!