One of the playgrounds near our house has an angled ring shaped merry-go-round:

There are lots of different toys at the park, but this one is different. It has depth.

It's a freely spinning ring, with a 10deg tilt. As you walk up, it spins you back down again. It has enough momentum, however, that you can get yourself to higher positions on the ring by working it back and forth. Because of the tilt, your speed changes as you move around the ring, and you have to adjust your angle relative to the ring as well. First is the challenge of staying upright as you walk at the bottom. Then the challenge of getting yourself to the top. Then the challenge of getting around it at different speeds, changing orientation, riding with other people, rolling on it, jumping, etc.

There are shallow games, like Tic-tac-toe, that are fun for a bit, and deeper games, like Go, that can be enjoyed for years. While I haven't spent years on any of these toys, this angled spinning ring has the most depth of any I've seen.