My indoor cat has a (rotating) variety of toys, things to climb on and scratch, and places to run around; I'm not concerned about him getting enough physical stimulation. But I've heard that cats also need mental stimulation. How do I best provide that, both when I'm playing with him and when he's alone in the house? I know that there are "puzzle toys" that reward solving the puzzle by dispensing kibble or treats, but I'd rather not have that happening all over the house (especially where any missed kibbles could be ground into rugs). What other options do I have for exercising his mind as well as his body?
The cat is probably 12 or 13 years old. Getting another cat isn't an option right now because of disease-transmission concerns. Other than being a carrier for something I'd rather not spread (coronavirus), my cat is healthy.