At the start I set myself a goal of flying between the airfields closest to each European capital. Planned it out simply in a notepad file with the airport codes and am working my way through it.
I sometimes pick random places to fly between on the map. Basically spin the globe, zoom in somewhere and choose a start point. Then head off in a random direction to find the destination airport (check how long it takes vs how long you’ve got) then hit fly.