We took a day trip to Penrith to just see the castle. The trip ended up being a bit longer than planned. It poured all day, so we had to postpone the castle tour, but spent the day in the numerous chanrity shops in Penrith, and having lunch in the little cafe at Sainsbury.
I thoroughly enjoyed the trip, don't know if the first part was worth it to the boy... He did enjoy running around at the castle.