Bats lend themselves to paper folding since their bodies can be easily reduced to flat planes. Here's a very simple version by Nick Robinson that you can fold in a minute or two. I followed this good video tutorial.
Picture this: you make a couple bats along with the video to memorize the steps. Then put on your favorite Halloweenish movie and fold an entire colony of bats. You could crank out dozens, hundreds! (Depending on how engrossed you get in the movie. Beetlejuice yields 65 bats. Batman Begins yields 2 bats.) Then tape strings of bats to fishing line or invisible thread and hang garlands from the ceiling. Or stick them to the wall in a huge group with a little ball of poster putty or loop of painter's tape at the bottom point of each bat.
Tips: If you end up with a little edge of white showing around the bat's head, color it black with a marker. Find packs of black origami paper for a few bucks at Amazon.