When I was a child, old ladies used to crochet bathroom floor mats out of strips of white plastic milk bags. I found these really ugly. Nowadays the selection of plastic bags has increased tremendously, all sorts of colours and qualities are available, even bags with colour blends.
I cut some bags to spiral strips 10-15 mm wide and used 4 mm metallic needles for knitting. Not hard at all. The most useful know-how learned, was to hold the strip folded in double, if the bag's inside colour would not fit the project. Some sample swatches:
Floor mats for bathroom or mud room would probably still be the best usage for plastic knits. Perhaps a shopping tote or swimsuit bag for a hiking trip? But seriously, nothing really pretty and cosy can hardly be brought about.