Work on actual puzzles. You could have a bunch of rooms that could actually relate to the gameplay rather than being behind the level. That's how you add challenge.
I was definitely inspired by Hapland on this game, so that's where I would look.
Thanks for all the comments. There will be a sequel eventually. And if anyone wants to know, there are 14 different potential "kills" on this particular puzzle.
 "JumpBox"){kind=icon}
 "The Pickle Buildings"){kind=icon}
 "Hoppin at the Avocado Condos"){kind=icon}