I went down here because Jon was having a go at land yatching thanks to his sister Anna. It is about 2-2 1/2 hours drive from Cambridge so I think that is a bit much to do in one day so we stayed in Canterbury.
Dover has a beautiful coast and Dover Castle on top of the cliffs.
Great destination for a weekend away. I would suggest combining this with a stay in Canterbury. While Dover is lovely, there is not a lot to it. So go to Dover for the day, then stay in Canterbury and do Canterbury the next day.
Make sure you get yourself some cockles!
All the Best!
American to Britain