There seem to be quite a few promising bingo squares (I've listed them all here:
http://themisathena.booklikes.com/post/1961746/discworld-october-group-read-starts-today ). In addition to the two squares you mentioned, I expect that the book will fit -- at a minimum -- the Supernatural, Creepy Crawlies, Monsters, Ghost Stories, and Grave or Graveyard bingo squares; possibly / probably also Spellbound, Demons, Relics and Curiosities, and / or Doomsday.
This will be the first time I'm reading it as well, though, so take the above with a tiny grain of salt. I've decided I'm just going to start it and then apply it to whatever seems to fit best.