Seeing as you're staying for 12 days, you may want to go to Maui for a bit, but don't go there every day. There's plenty to do on Molokai, including simply relax.
Here's a trip report I did two years ago:
http://www.flyertalk.com/forum/showthread.php?t=181847
A whale watch trip may be an option toward the latter portion of your stay. You're going right when whales typically start to arrive.
Be sure to go to Kalaupapa, the leper colony.
There are also a lot of good hikes on the island. If you like hiking, pick up a guidebook. I picked up one at the pharmacy/five-and-dime in town.
Here's another thread that might be helpful:
http://www.flyertalk.com/forum/showt...hlight=Molokai