Sunrise/sunset maps based on terrain obscuration
In summer or winter, having sun or not makes a huge difference for many activities, particularly in the mountains. Climbing, cross country-skiing, camping might be much more/less fun/safe depending on when the sun comes in. I once wanted to go climbing in winter on a south facing crag (Lidernen area, central Switzerland) but had to find out that the terrain was sloped in the wrong direction and at that time the climbing sector would never get sun.
So the idea is: Based on date/time, calculate a map (overlay) that shows where there is sun at that time and where a mountain is still in it's way. This requires some basic ray tracing and a good enough terrain model. I don't know if the SRTM data is sufficient, but mm resolution LIDAR data gets availble progressively so it is just a matter of time until each mole hill is mapped. Most people will have DEM data downloaded already for hill shading, so this should not be a big thing!
Instead of a black/white map overlay, it could also be false color indicating the time of sunrise/sunset for a specific location.