One interesting thing about Dover is the port. It's easily visible from the cliffs and the castle, and it's huge. The sheer volume of lorries (and cars) arriving on land and ferries arriving and departing by sea is fascinating. It's like watching clockwork.
It's easy to see why it's so busy - France is clearly visible on the other side of the Channel. And not only are there tons of ferries going between Dover and Calais, but lots of other traffic just passing through. I'm probably geeking on this somewhat, but I found it fascinating.