It is possible to offer your guests free items during their reservation process in the Booking Engine. 

In order to create such extras, first, create products in Item Manager. Check this instruction.

In Edit Sale window, select the checkbox Free Item (it will deactivate the prices fields). You can also add an image to the product and a description - it will be displayed in Booking Engine:

Next, add that item to extras in Property Settings / Booking Engine / Settings:

The guests will see the product this way: