Mallorca is a good training destination for late fall and early spring. Since Mallorca is a famous summer destination it has a good infrastructure on hotels, enterainment, restaurant bars and is reachable by cheap flights.
The Trans Tramuntana is a classic. The Traumtana mountains offers a lot of trails for a multiday trip to explore the island and it's hidden jewels in a different way. The highest peak is the Puig Major with 1.445 m.
Most areas of the island offer very good mountain biking potential. In particular Sa Comuna near Bunyola & the Soller valley in the west, Cala Millor on the east coast, Pollenca / Alcudia in the north and Peguera in the south west. The west coast is very mountainous with bigger climbs and exciting descents. The east coast is generally flatter but still offers excellent trail opportunities.
For the less experienced biker, one good area is Na Burguesa near Palma which has plenty of forestry roads. Sa Comuna near Bunyola is also a good place to explore with many 4x4 roads in lovely forest comprising Mediterranean Holm Oak and Pine.
Finding the best routes in the past on Mallorca was notoriously difficult. However, a guidebook is now available with up-to-date route descriptions, maps and GPX downloads. The guidebook covers the entire island and is a perfect accompaniment to a mountain biking holiday. It is available as a download or hard copy from Rock and Ride Mallorca. All of the main biking areas are also well served by mountain bike hire shops, some of whom run guided tours.