I always take pictures of areas I walk or cycle, which is very helpful for upating OpenStreetMap. Here’s a summary of the current surveys in the forest.
The most concentrated area is probably near Bambësch, a forest located North of Luxembourg City which is almost on my way to work and is worth a detour from spring to autumn a few times a week:

I still have hundred of ways to visit, fix or update in the eastern part of the forest; which will make my detours longer soon.

The good thing is that each time I go out, I have fewer waypoints to check; last year for each waypoint checked, I added five new points. Sometime I forget to remove points from my waypoint list which make me revisit area already visited :). Also, I added some ways and points tagged with fixme or weird values to my to visit-list.
Over the last year, I’ve been walking and cycling thousands of kilometers; I should check my GPS logs to get the real figure.
This last picture shows area I’ve been exploring near Luxembourg City.

And finally, my OSM Heat Map