This Dec. will be my 3rd trip to Jamaica, mon. LOVE IT THERE! We always travel to all-inclusive resorts. You can go as cheaply or as expensive as you want, depending where on the island and what resort you choose. If you are into a more laid-back irie atmosphere, as I assume you are by your name, Negril is THE place to be. In fact, it used to be the destination of choice for hippies back in the 60's and 70's. No buildings are taller than any palm tree, by law. People are friendly and the Red Stripe is cold!! Keep in mind that it is about a 1.5 hr. drive from Montego Bay airport, but your bus driver will stop for beer or get you beer before the journey. Go to your local travel agent and check out deals from Apple Vacations or Funjet Vacations. Apple & Funjet also have good websites on their vacation packages as well. KEEP IN MIND.... you get what you pay for. Don't go too cheap as you will notice it at the resort, ie. 600 rooms, and only 1 bar and eatery, etc.... keep in mind room to bar/resturant ratio.
If you go APPLE vac. don't stay at less than a "4 apple" rating. Same for Funjet. 4 stars and above are usually pretty good.
Don't know about the mountainbiking there, but I've read about it online. It is available, but you will be so relaxed that you may not want to do it. Besides, the mountains and heat may make you think twice. Wouldn't a dip in the pool or snorkeling be more fun?!?! Parasailing, waterskiing,