There was a YouTube video I saw a while back of a really cool, homemade brain game. Basically grab a few plastic bottles, put holes through them so you can put a rod through (put the holes high up enough that the bottles will naturally want to rest with the top upwards), mount the rod on a basic stand so the bottles can flip freely, and fill them with treats. The dog has to try flip the bottles to get the treats out, and usually they won't fall out if they just whack it with their paw, they actually need to try hold it upside down. Probably not all that challenging, but it's a pretty neat DIY idea I thought, and can be made with stuff most people have laying around