may I suggest this toy?
He discovered it in the toy cabinet on Wednesday, and played with it for 20 minutes. Then I needed to run upstairs, so I placed it with him in the playpen for another 15 minutes. Then he cried when daddy took him out of the playpen because he had to leave the toy as we were off to run errands. Later in the afternoon, I placed him back in there with it, and he played in focused concentration for 45 minutes.
And he is playing with it now, still determinedly trying to work those locks as I type.