The Mountains to Sea Trail is a long elevated walking route across Xiamen Island, running for about 23 km from Huweishan Park toward Guanyinshan Beach. It gives you a way to see the city from above without committing to a hard mountain hike.
You do not need to walk the whole route for it to be worthwhile. Pick a section near the lake or one of the hill viewpoints, bring water in warm weather, and use it as a quieter counterpoint to Zhongshan Road and Gulangyu.