You can create these add-ons by using the form fields on the options that you create.  These add-ons can be price check boxes that the customer can click and add to their booking.  You can find out more about adding these to your inventory options in this article:
https://www.rezgo.com/support-article/add-additional-booking-fields-options
If you need more details you can also refer to this getting started video:
https://www.rezgo.com/support-article/getting-started-4-add-custom-booking-fields-options