That is a great day out! For longer, in terms of rail interest, I'd suggest somewhere along the North Wales coast. It's not so busy as it was, but you'll see Gerald the WAG express, Virgins to and from Holyhead/Bangor, and the local services. My own personal preference would be Llandudno or, even better for the trains Colwyn Bay. I;d also take trip down to Blaenau Ffestiniog, stopping off at Betws y Coed for some chips!