Unlike other computers with which you may be familiar, that support multiple user profiles - other than “managed” educational environments, OS/iPadOS devices are not intended to be shared.
Assuming that you have sighed-in to iCloud from iPad settings with your own AppleID, your activities should not be visible to the owner of the device.
Assuming that the owner signed-out of iCloud - and you have signed-in - the owner will not be able to change back for 90-days. This restriction, intended to prevent account abuse, cannot be bypassed.
So as to preserve your privacy, before returning the iPad to its owner, you should properly prepare the iPad:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support