You can make all sorts of designs and such with them! In my world they even have pumpkin carving competitions nowadays for a large sum of money. It's...quite fun, especially if you do it with your friends or family. But I believe the pumpkin was chosen because of it being in season, yes. Typically it was gourds, squash and a variety of other things back in the old days. There's also the, ah, will-o'-the-wisp phenomena that made them quite popular to use!