Keeping ferrets happy is a never ending task. Toys that challenge them both mentally and physically are hard to find.
One of the best toys I've discovered recently is this treat puzzle toy!
It has varying levels of difficulty - the center white bone pieces can be removed, so that just the red ones slide left to right. So the easiest level is just to hide treats under the red pieces.
Next is that the red pieces also are hinged. It's a little tricky for ferrets because they only open in one direction. Mine could smell the treats and knew that they were inside the red boxes, but it took a while to learn how to open them. It's something they still struggle with a little bit (but that's a good thing - if it were too easy they'd get bored quickly).
And finally, the most difficult thing is to put the white bone pieces back in and put treats underneath them. It's not easy for ferrets to remove these. In fact so far none of mine have been able to, but I'm sure they'll figure it out eventually
The best part is that you can hide treats in different areas every time, so that they always have to do a little work to get them out. So far it's been a couple weeks and the toy hasn't gotten old yet. They get excited and run for it as soon as I let them out and start sniffing and nudging the pieces around.