The top floor is open and breezy, far from the crowds, and is the best!
I thought the 2,200 yen admission fee was a bit expensive, but I thought I might as well drop in since I was there anyway, and it turned out to be much better than I expected.
One floor down from the observation deck, there is a corner where you can fold origami cranes and play some games. What's interesting is the attempt to post a cranes you've folded yourself. It's best to experience this for yourself.
It's very close to the Atomic Bomb Dome, so it's easy to stop by!