Depends if you like bumpy and narrow, if not stick to the main course. Can't really go wrong anywhere out of the towns though Peel to Kirkmicheal and Douglas to Laxey spring to mind, some of the others are a right roller coaster of a ride, great fun.
Great ride down to the calf of man and a great cafe there too then up to peel loads of great roads just have a wander the island is small enough not to realy get lost Even better if you have an enduro bike get out on the green lanes when the racing is on and find viewing places no one elce can get to