You are going to want a lot of pumpkins seeds, around 10 per party guest. You can save the seeds from the pumpkins you make into Jack-o-lanterns and dry them or buy them from the grocery store.
Use a string or tape to make a starting line for people to stand behind while spitting. The first contestant selects three seeds from a bowl and attempt to spit them one at a time, as far as possible. Repeat the process until all of the guests have had a turn and then the first person starts the process again. Each guest should end up getting three turns of spitting three seeds so nine total attempts.
A judge tracks where the pumpkin seeds land and keeps up with who currently has the longest attempt. Have a tape measure handy so you can measure the distance if you are not sure if one attempt is longer than another.
Once all guests have had their turns the person who spit the seed the longest distance is declared the Halloween pumpkin seed spitting champion and awarded a prize. If you have kids playing you might consider awarding separate prizes for the youngsters.