If no children are doing this - use a knife if it is easier for you. I have been expiramenting with the best things to do with them after they are cut out. Glue and glitter always work for me. I tend to like colored glitter on the unpainted styrofoam and clear, gold or silver on painted or colored foam. You can spray paint the styrofoam, but do it either well ahead of time or after you are done, as it will take a little while to dry if you use too much paint.
Kids can paint it with washable paint and add glitter while the paint is still wet.
Use clear glitter and light brown paint to make it look like gingerbread. Markers are NOT a good idea, it comes off and doesn't dry for days. Crayons do work, but the styrofaom eats the crayons rather quickly. Be creative with this one. Glue on little notions.