Here is a great post with a few ways to use an unripe watermelon, including an Indian Salad, a puree for mixed drinks and a sorbet:
http://portpotager.blogspot.com/2012/08/what-to-do-with-unripe-watermelon.html
If you're into pickling, you can also try out some pickled watermelon rind recipes:
https://www.tasteofhome.com/recipes/watermelon-rind-pickles/
https://altonbrown.com/recipes/watermelon-rind-pickles/