Zuzu,
as you know, at the height of summer at high latitudes, the sun circles slowly to the horizon. So if you watch the sun set from a west-facing harbor and can quickly drive up a tall enough hill that looks westward over the same large body of water, you can watch the sun set a second time that day. I guess you could do the same with sunrises/hills/eastward bodies of water. Probably most of the fun is in being able to say you've done it...
