Fabulous commenter Pixel Pixie pointed me to these instructions for making a very cool origami bat. (I commented that I wondered if the folds would be suitable for a fancy dinner napkin and she replied, “One way to find out: vampire dinner party. BOOM.” I like the way she thinks.)
This particular fold is a little bit fiddly, but it’s fairly realistic as origami goes; many of the simpler designs are sort of, “It’s a bat! Or maybe a crane! Would you believe a pigeon?” However, it’s not so complicated as to be completely beyond the amateur origami-ist, so with a little practice you should be able to turn out flappy paper bats on demand. A miniature one would make a cute brooch.
Upon closer examination I’d say it’s definitely too detailed for a napkin fold, but fortunately googling “origami bat” turns up loads of easier designs: This one is nice (although the napkin would have to be pretty stiffly starched), and these look especially straightforward. A couple of others worth noting (probably a tad too complicated for napkins) are the flapping patty bat and this compact design.
The same search also turned up this list of Halloween-themed origami, so you can expand into folded ghosts, witches, spiders, and even tombstones. Some of these would make really interesting placecards or gift toppers, and since all you need is a piece of paper you can practice your craft nearly anywhere.
(Thanks, Pixel Pixie!)