Permanent or temporary? you have to be careful though, if you do too much, it can knock the fan off balance. You might be able to paint spider webs on the blades and find spiderweb globes. You can also put orange and black lights in the light kit if it has one.
You could get black or white streamers and run them from the bottom center of the ceiling fan(so the blades are above it and unaffected) to the side walls of the room and make a big spiderweb. Then have a big spider on the bottom of the fan in the center of the web.
You could always get some black construction paper and cut out some bats, spiders, etc and tape them to the blades, or suspend them with thread or fishing line. But you may have balancing issues if you put a lot of stuff on and run the fan.
You could make some small lightweight ghosts using cotton balls and tissue, or ping pong balls and creepy cloth or cheesecloth. There are numerous tutorials that may give a better/lighter/easier idea or variation. Suspend them from the blades with fishing line and leave the fan off or maybe on slowest setting to have them "fly" around. You do have to worry about blade balancing, but if you measure out the same materials, IE 3 cotton balls and a tissue with 6" of thread to tie it and 12" of fishing line to hang it, weights should be pretty close between each ghost.
Really, we don't know what other decorations are in the room, and so that may be "cute" ghosts among other scary decorations, and so don't match the theme or feel of the room. Are you still wanting to use the fan or will it be off? Are you doing an event, where you could have a one-time effect happen? You said it's plain jane. Which means people won't notice it among other decorations. You could put a bunch of small spiders on top of the blades and at an opportune time turn the fan on and rain down spiders. Or maybe hide a dropping spider on the far side from where you enter the room so it goes unnoticed.