the
Bergan Turbo Track Cat Toy is definitely one. This toy reminds me of a child's toy railroad tracks, because you can double it or add separate tracks to it to make 'infinite' fun for your cat.
Speaking of 'infinite,' I'm partial to the infinity shape for the Turbo Track because I think it's the most challenging arrangement of the kit's parts but, as you see in the photo below, you can arrange several shapes for variety.