understand the business and line you up with a good liability policy that doesn't have a million exclusions in it that would negate coverage if it's ever needed. Then buy a decent umbrella policy (minimum of $2 million, probably $5 million is preferred). Umbrella insurance is cheap, so get good umbrella coverage on top of a business liability policy.

Do an LLC and segregate the business assets from your personal assets as a second layer of liability protection.

As a third layer of protection, have all your rental contracts include a liability waiver and hold harmless/indemnification clause where the renter waives any claims of liability against you and agrees to indemnify you and hold you harmless for any claims made by any of their guests who might get injured on your equipment. Don't borrow someone's form from online. Hire a competent attorney to draft your agreements for you.