I have tried many of the store toys on Grimm, but he prefers the stuff I have made him at home, really.
Hay ball: I got a whiffle ball (the hard plastic kind with slot shaped holes in it) cut the holes to be bigger, then stuffed it with hay. He loves to grab/chase/throw it.
Hay box: I took a heavy garbage bag box (the kind where the opening still has the perforated bit that you have to push out to get the box open) and filled it with hay. He loves to drag and throw it. He seems to really prefer pulling the hay out of something that gives a little resistance, like this and balls. That perforated door gives it enough resistance that he likes it
Monster box: I was feeding him greens on a mat, but he started peeing there, so we came up with the monster box idea. It’s a small cardboard box with holes cut in the sides. I fill it with greens and treats, then tuck the top closed. He loves to drag/throw it around and also resizes the holes to his liking. I also add paper shred to it and let loves to drag all the paper out and make a huge mess
Carrot Pole: we have cats, so we have those cat fishing pole things that you put feathered “birds” on. I repurposed one to hold a carrot and will randomly place it around the house. It’s high enough that he has to work a bit on his plan of attack but low enough that he can reach it without jumping.
Good luck to you, I hope you have as much fun trying to challenge your bun as I have!