1. Solo cups will transform you into a Lego:
Just paint a box and cut holes for your head and arms, then glue the cups to the front. Write "LEGO" on the cups if you really want your friends to get it without thinking.
2. Use a paper bag to make a safari vest:
And cut up a paper towel roll for binoculars; full instructions from Real Simple. The example shown is for a child, of course, but you could size up for an adult. Warning: costume not ideal for rainy weather.