The biggest "wow moment" for us in our recent trip was the Sagrada Familia in Barcelona. I realize "everyone" does it, but I myself have never seen anything like it (in many trips)--and when I walked in I remember actually saying "wow", more than once.
We're in our early 70's and actually survived walking down the seven hundred narrow steps (the security person said seven hundred and I believe it) to get down after taking the elevator trip up the Passion Façade tower. Apparently there's no other way to get down. Security politely asked us to demonstrate we had the necessary agility (jumping up and down) before allowing us to go up the tower--and we were by far the oldest people we saw in the tower.
Having done it, I could easily imagine someone with a problem with stepping down hundreds of steps being injured.
If you look old enough, I'd expect that security will ask you to physically demonstrate that you have no mobility difficulties (we had to jump up and down several times, and apparently a videocam system captured you doing it, just in case something happens). If you don't pass the agility test, they apparently refund the tower portion of your entrance fee.