Ancient ruins can be found in pretty much every other part of the globe that is sufficiently populated today. Another exception maybe Australia.
There are many ancient ruins in North America. Some of the more famous are Cahokia Mounds, Mesa Verde, Teotihuacan, and Chichen Itza.
Here are a couple of previous threads that address this question:
The American Southwest/Rocky Mountain region has ancient ruins all over the place, depending on your definition of ancient. It's impossible to hike in the canyons behind my grandmother's house in Eastern Utah without encountering dozens of old Native stone structures, which I'm told were primarily granaries. Not to mention the petroglyphic art that covers the cliffs.