I have been doing something a little different for quite a few years with all the plastic grocery bags that I get. I save all of them in a large plastic bag, and then in the fall use them to make outdoor decorations for my yard for Halloween.
I stuff a large black plastic contractor bag with the plastic grocery bags to make a nice rounded life-sized grave. To make monsters for the graveyard, I stuff grocery bags in tall kitchen (or similar) bags to make a soft (and moldable) but solid-looking chest, lower body and legs, and stuff them in two plastic newspaper bags for each arm. All are covered with old worn-out clothes and boots. Stuffed rubber or plastic gloves make the hands. A larger plastic bag that you might get at a department store can be stuffed with the smaller bags to make a head for the monster. (Just put a plastic or rubber mask on it for the face.) The monsters can sit in the graveyard on a chair or at the back of the graves (propped up by a stake behind them), or can hang from a clothesline stretched between two trees.
You can also stuff the bottom half of a tall white kitchen bag with the grocery bags, tie it tight at this point, and make round eyes and mouth on one of the stuffed sides with a black marker or black duct tape. When hung by a string from the bag bottom (now the top), you have an excellent ghost.
The nice thing about using these plastic bags for stuffing is that, even if it rains around Halloween, only the outer clothes of the monster get soaked, so it is easy to dry them out after Halloween and store them away until the next year, without having to worry about the inside stuffing mildewing. Since I live alone, it takes nearly six months to collect enough bags to make one new life-size monster or one new rounded grave. Over the years, I have developed a rather large graveyard populated with monsters and ghosts at a very low cost using this method.