MTB (Mountainbike) is a sport that the last few years really got many Danes out in the nice weather, MTB is a sport that can be played by anyone who can ride a bike, of all ages. MTB is both the term for the type of bike you use but also the term for the bike ride itself, an MTB ride is not a quiet ride on a straight road, but rather mostly on winding and muddy trails.
Bornholm is perfect for people who love mountain biking, with the many different trails and altitude differences that especially the northern part of Bornholm provides, is perfect for an MTB trip.
Bornholm has an annual MTB event called Stage Race Bornholm (http://www.mtbstageracebornholm.dk/), the mountain bike race is for recreational and elite MTB riders.
If you want to ride on official tracks, there are currently two good tracks in Almindingen. One track is 2.7 km long and is called a cross country track. 2.7 km is not much for those who usually ride elsewhere, but it offers good challenges for all levels. For those who like more speed and more technical challenges, the downhill course is a great ride. 500 metres downhill through Almindingen
You can read more about both here: http://naturstyrelsen.dk/naturoplevelser/aktiviteter/mountainbike/almindingen-bornholm/