I've heard Schlendern is close but I'm not sure it captures the essence of wandering or exploring like the french word Flâneur.
In my opinion, a good verb to describe what a Flaneur does would be lustwandeln, though it's not that widely used (anymore), but there is no according noun.
Then, there is Spaziergänger, of course. This kind of person can do anything from flanieren (in a very relaxed manner) to more ambitious walking (though not really intense hiking). The verb is spazieren gehen, and the activity itself is called Spaziergang.