1/ best setup Roku account (s) for "renter" boxes separate from your own. You can do this without a credit card, just need different email addresses (easily available for free). To manage the device, just log into relevant roku account using correct email (use different passwords on each account too).
2/ read up on guest mode. Maybe set up devices in guest mode before handing keys to renter.
Equipping rentals with Roku TV I get. Devices? Not so much, just invite renters to bring own device and equip TV with HDMI and power extension cables to make plug in easier.
Not a Roku employee