Jen, one of our staffers at the Lion Brand Yarn Studio, adapted our Cream Circle Washcloth pattern to create a bathmat. The original pattern used Cotton-Ease® (a worsted weight yarn) and a size H crochet hook to produce a washcloth that measured 5" in diameter. But after seeing our new Zpagetti (a super bulky weight yarn) hit the shelves, she had to try it. Simply by using this yarn in conjunction with a larger hook (size Q) She was able to create an adorable bathmat.
Jen followed the pattern almost exactly (only substituting single crochets for slip stitches in the final row) and used two cones of Zpagetti yarn. Once you become comfortable with substituting yarns of different weights you might want to try it on garments as well. I suggest you start by reading our 5 Articles to Help You Make a Sweater That Fits & Flatters article.