Ideally, every other category should be a subcategory of at least one other, and every article should be in at least one category; many will fit well into two or more (e.g. location and subject matter). A full list of existing categories can be found at Special:Categories.
To add a new category, just add [[Category:Categoryname]] to the article or existing category page (after checking whether any existing category is suitable).
This category has the following 33 subcategories, out of 33 total.