A quote in "Huckleberry Finn" that shows the transcendentalism idea of simplifying life?
As you say, the transcendentalists, like Ralph Waldo Emerson and Henry David Thoreau believed that people should simplify their lives. They thought that people got too tied up in loving their material possessions until they existed for the benefit of the possessions rather than the other way around.
The Transcendentalists also argued that people should behave in the ways they think best. They said people shouldn't just blindly follow what society says is right.
Both of these ideas are present in The Adventures of Huckleberry Finn. Here's the quote that I would pick to show these ideas. It's from the very beginning of the book:
but it was rough living in the house all the time, considering how dismal regular and decent the widow was in all her ways; and so when I couldn't stand it no longer I lit out. I got into my old rags and my sugar-hogshead again, and was free and satisfied.
I hope that helps.