The nettle is a great suggestion. I'm sure anyone would be glad to have someone pull nettles for them. They litter the river sides and creek beds around here and are a most hated weed. I was actually just reading about them in a book I have, Native American Ethnobotony. Of the 4029 plants listed stinging nettle comes in at number three with the most uses across drug, food, fiber, dye, and other uses. It has 222 noted uses by native Americans. 36 of those for fiber.
Speaking of, the book is great for anyone interested in native North American plants. It a great resource for foraging for food or any other use while out and about. The only issue is it certainly isn't a field guide at around 1000 pages and the size of a large texbook. There aren't any photos or illustrations either, but that just indicates how much information is packed inside.
Edit to add: Nettles go along great with the concept of the naturally scavanged built canoe. Along with cottonwood, willow, and pine sap for turpentine or fish gut, all of the materials can be found and harvested right along the river.