I’ve been using the Lenape Talking Dictionary to learn Lenape. It includes lessons. I’m also learning Navajo on Duolingo.
The easiest American Indian language for adult learners is said to be Quechua (except it’s South American, sorry). Its grammar rather resembles European languages.
Salishan languages have complicated sound systems. As for Uto-Aztecan, there’s a song in Yaqui I’ve been trying to find out more about.