Pages vs Categories – help?

You can only set categories for posts, not for pages. Categories are topic areas you can associate with different posts (only one category per post). So for your Travel section, if you use posts to create your content then maybe you would have categories such as South Asia /South East Asia / East Asia / Australia and South Pacific . Then, if you have a list of your categories in your sidebar (created easily with a widget), your visitors can have another way of navigating through your content.

If you created a Travel post (https://yoursite/travel/), with your list and brief descriptions of destinations as you mention, then you could link through the images and text to each destination page:
https://yoursite/new-zealand/
https://yoursite/australia/
Both of these posts could have the Australia and South Pacific category (created and selected on your edit post page).

Your other option is to create pages. In this case your countries would be sub-pages or children of the parent page. e.g.:
https://yoursite/travel/
https://yoursite/travel/australia/
https://yoursite/travel/new-zealand/

Using pages gives you a different hierarchy as you can see, and you could link to these sub-pages as above (as per your list and linked images/descriptions on the Travel page), and show them in a drop down menu below your main menu Travel tab. But, you cannot assign categories to pages.

Sorry if this is a bit or ramble, but it makes real sense to be sure of your site structure before you begin. Here’s some more info:
https://www.wpbeginner.com/beginners-guide/what-is-the-difference-between-posts-vs-pages-in-wordpress/
https://www.wpbeginner.com/glossary/category/