Add custom fields to them as per your business requirements. For example: You may be providing pictures during the trip, or drinks and snacks. Now you can mention them in the booking form itself and charge them differently.
Cross Sells & Addons:
If you provide on board breakfast, snacks or drinks as an extra option, you can create Addons for them, and rate them separately which will be shown as an option on the storefront. Once the customer chooses an extra service it will be added to the total amount of the booking.