Since I've already mentioned Yugoslavia and Yosemite, that leaves me with Yellowstone National Park for the letter 'Y'. The Park was created by an act of Congress on March 1, 1872.
I've traveled through Yellowstone from both the North and the South entrances. This park is beautiful. It has everything you could ask for in a nature adventure.
There is water, both flowing and falling:
There are wild animals, like this elk,
or bison just walking along the highway, right next to your car:
There is no shortage of beautiful vistas:
Of course, everyone is familiar with 'Old Faithful', but there are many more wonders to see when it comes to geysers and hot springs.
Go West young man or woman. See Yellowstone.