There are also over 10,000 state parks in The United States.
The United States of America is filled with amazing outdoor adventures, and while this guide mainly focused on National Parks and gateway cities, there are countless other national and state forests, national monuments, wildlife preserves, national historic trails, memorials, and recreational areas. There are also over 10,000 state parks in The United States. There is no need to wait until you have a buddy to travel with--solo travel is an incredibly liberating, freeing experience.