I think York is an excellent idea - there is a roman museum with an underground boat trip, isn't there? Also the minster, Betty's TeaRooms, and the possiility of lots of other nearby places - harrogate? is the Yorks Sculpture park within local train distance, because that is truly fab. Also a coat trip by train might be a possibility from York
We've just been away for the week and took a day trip to Bath and I really loved it. The boys enjoyed their trip on the open top bus (which has a stop at a HUGE park) but were too young to appreciate much more (age 4 & 2). If I had the chance I would definitely return it was a great place to visit IMO.
And the time after that, make it York. It has a magical atmosphere and my 6 yr old never tires of it. We had a great boat trip, really cheap, and there are loads of museums for all ages, walking around the walls, great parks, usually some street entertainment, Waterworld on the outskirts and lots of other days out within 15 - 30 mins.
more tomorrow - to bed.