Hi, I've never heard of cats eating plastic bags before, cats often have a certain thing that they like the feel of, and him eating it may be a wild instinct? one of my cats will only play with crisp packets folded into a triangle, and my other cat loves stealing sponges, I often find them on the floor. You may just have to make sure you don't leave anything lying about and try and keep him occupied with other toys that he likes. To stop him drinking from the toilet just keep the lid down, maybe let him drink from the tap? my cat loves drinking running water from the tap and from a catit water fountain.

Hope this helps.