If trail running isn't interesting or challenging for you, why not try skyrunning? Trail running is where you run on trails through natural areas instead of on roads or footpaths in built-up spaces.

A more official definition would be from the International Trail Running Association: “... trail-running is a pedestrian race open to all, in a natural environment (mountain, desert, forest, plains) with minimal possible paved or asphalt road (which should not exceed 20% of the total course). The terrain can vary (dirt road, forest trail, single track) and the route must be properly marked.”