Got a bag of old shirts? Cut them up and make a rug!
My friend was getting rid of years' worth of her son's sports t-shirts. They were all youth medium and large, in preshrunk cotton. I snatched them up because they were in bright colors, with the majority in red and black and no whites. I knew they'd be perfect for a bathmat-sized rug.
I cut in spirals, starting from the bottom and working until the armpits. I wasn't worried about the width being consistent or cutting out screenprinted designs, because these small variations add a homespun charm to a throw rug. You could rotary cut strips from the shirts if you like a neater final product. The amount of yarn that can be collected from each shirt isn't much, so you'll need at least a dozen for a bathroom size and a whole collection for anything more ambitious.
The way you weave the rug is also up to you. Many examples online go in a circular pattern, but I went with plain rectangle half-double crochet rows so I could sit it at the base of the stairs. I especially like the extemporaneous effect of Little House in Colorado's design. Sometimes it takes more reserve to keep a craft simple. As a bonus, she also has a tutorial for a matching basket.