Recycled art can be a wonderful eco-friendly decoration for your yard. If you do melt plastic, you will want to be in a well-ventilated area and possibly wear a mask as plastic can release harmful gasses when melted down. You will also want to use a non-spray paint and glue for the styrofoam, it can melt as well (Design Master Spray Glue does work with Styrofoam brand foam). For coating with glitter, check your descriptions before purchasing and choosing one, make sure it works with your type of glue, you can also find ones that can resist fading much longer outdoors.