Here is a list of destinations from South America, Europe, Asia and Australasia which are less-visited by international tourists. Some are likely to be completely unknown, whilst others do get some international tourists but are overshadowed by more famous destinations within the same region or country. For example, for South America I have avoided places like Cusco, Machu Picchu, Quito, the Galapagos Islands, Lake Titicaca, Salar de Uyuni, Buenos Aires and Rio de Janeiro. Note that some places might be pretty popular with domestic tourists, but aren't as much visited by international tourists.
To narrow things down a bit, I've only included places I've personally been to. I'm from the UK, so about two-thirds of places here are from there - I've put these at the end. If you haven't been to the UK, you can stop scrolling after New Zealand!