Here’s a feline-based subcategory of objects on Etsy: Cat bridges.
These seem like they’d be a fun DIY project for parents with engineering-minded kids.
For cats who get bored easily, an alternative is to build them an entire climbing facility, as DIY’er Vivianne did:
Lest you think these were all constructed by outlying weirdos, it’s worth noting that the feline toy market is valued at $3.3 billion and is expected to grow to $4.5 billion in the next ten years. Those Etsy sellers are probably doing brisk business.