Bymarka is Trondheim's outdoor back garden: a large area of forest, lakes, ski trails, and hill paths immediately west of the city. It is one of the easiest ways to understand local life in Trondheim, because residents use it in every season rather than saving it for special outings.
In summer, go for walking, lake views, and a break from the compact centre. In winter, the same area becomes cross-country ski country. Public transport can get you close to several access points, but choose a route before you go; Bymarka is large enough that wandering blindly can turn into a longer day than planned.