If you are flying to Barbados on a one-way ticket, sort out the return journey before you travel. Barbados' official immigration guidance lists a return ticket to your country of residence among the items visitors should have ready. That is more specific than the general phrase “proof of onward travel,” and it changes how you should plan a trip that includes other Caribbean islands.
You can still build an adventurous itinerary around Barbados. Just avoid treating a short hop to another island as automatically equivalent to the return flight described in the country's visitor guidance.
The difference between leaving Barbados and returning home
Picture two travelers arriving at Grantley Adams International Airport, the main international gateway to Barbados.