If you love history, you will love Rochester.
Not only does it have the Cathedrol which was built in 1180 and the castle which goes back to 1087 and the Rochester Guildhall Museum but it was also home to Charles Dickens.
The beautiful old buildings with amazing architecture, there are lots of shops to buy trinkets and souvenirs, art work on the side of buildings, cafes and pubs.
It is free to get into the Cathedral but make sure you check the times of services so that you do not disturb a service. It is also free to get into the museum which is a brilliant place to check out all the local history.
You do need to pay to get into the castle if you aren't an English Hertiage member, here is the link for the Prices. You have to be careful when walking around as a lot of the building has crumbled away but it is amazing to walk around for the views and walking through history.
If you visit during the May bank holiday, you will be lucky to be around for the Sweeps festival, there is live music everywhere you go, dancing, food stalls, historial perforamces and just general fun for the whole family. Parking isn't great so getting the bus or train would be a lot less hassle.
If you go to Rochester before Christmas, you will have the joys of the Dickens Festival which is a huge Christmas market. There are parades int he streets, actors dressed in Dickens era clothes, street performers and food stalls. The Christmas market is in the castle grounds and has all the traditional stalls for Christmas items but also lots small local businesses.