Since Duolingo is only offering Navajo at this time as its only language spoken by a quotient of Indigenous people in the USA, there are thousands of Indigenous languages all throughout the Americas that are worth exploring.
One site in particular offers an excellent source for 83 Indigenous languages, mainly in Canada, which can be found on the First Voices website.
There, you can obtain written and audio examples in many categories. Some language learning portals offer songs, stories, and games.
Note: Some languages are closed to the public. and require registration.
