I made these sweatpants for my sister for Christmas. I used a heather gray sweatshirt fleece (that she picked out), some twill tape for the drawstring, and some 1 1/2″ wide elastic. I used a regular slacks pattern as a base (to get a nice, more modern shape than most sweatpant patterns will give you). I sewed everything possible with flat-felled seams, and just used my fake serging stitch on the rest.
Sorry for the pictures- she was not around to try them on, so I had to use the mannequin (not really good for pants).
pretty seams- hemmed with twin needles, side seams are flat-felled