We also explored the castle and took the cable car up the mountain to a playground built into the side of said mountain. Every afternoon was spent in the hotel pool. The boys never wanted to get out, even when they were long past wrinkly! At least this tired them out and they slept well :-)
I think the highlight for the boys might have been the car ride and especially the car train. My lowlight was scratching/denting the bumper whilst taking a tight mountain-y corner, then realizing I hadn't opted for the extra insurance. It turned into a more expensive family holiday than planned, but worth every penny.