Castle Acre is a rare Norman settlement. It has a Castle, Bailey Gate and Priory.
There is an English Heritage priory located here. The priory is one of the best preserved and dates back to 1090. It is one of the most complete that I have sen so far. On the website there are a few activities you can print out for the children before you go.
I think you could spend a good half of a day here seeing the priory and castle, going around the village, and having lunch in the local pub.
Not too much to organise and a nice place to get away for the day.