It's difficult to find a set of games more perfectly suited for Halloween than Costume Quest and Costume Quest 2. The first centers around your team of characters trying to restore the stolen candy to their neighborhood and rescue a kidnapped sibling, while the sequel has you fighting a team of dental-hygiene fanatics intent on ruining Halloween for everyone.
They are set up as RPGs, with sidequests and turn based battles. However, your gear is more than just what keeps you safe. Why is that? Because your costumes allow you to transform into what they represent, be it a knight, robot--you name it. The games look adorable as well, with a charming mix of spooky and quirky and environments that can't fail to put you in the Halloween spirit.
Plus, until November 1st, both games are discounted on Steam: $0.99 for Costume Quest, $5.24 for Costume Quest 2 or $4.99 for a bundle with both. Note too that the DLC for Costume Quest comes bundled with it.