Turnip. First get a turnip. English people will call it a swede, but it isn't; they're just misled. Seamus Kennedy will back me up on this.
Turnips are important because they give the requisite burnt turnip smell when you light the candle in them, and that smell is a hugely important part of Hallowe'en.
Once you've obtained your turnip spend several wrist-jarring hours trying to chisel out the insides. The pain is worth it. Besides, the blood makes it look spookier.
Carve pointy teeth and evil eyes and a skull-like nose (i.e. none). Light stumpy wax candle and put it inside. Hope that there's enough oxygen coming in through the holes you've carved to keep the flame from going out.
Sit back and enjoy the aroma, and wait for compliments and/or remarks on how "it's not a turnip, it's a swede, and duh, pumpkins are easier to carve".