If you know just about any game from Double Fine studios...you're going to be familiar with the healthy doses of creativity and humor, the studio is famous for.
Costume Quest is very straight forward combination of fast paced action-style rpg combat mixed with exploration. The premise of Costume Quest is on Halloween night a pair of siblings (you get to pick who to play as) who just moved to the neighborhood go out trick or treating however candy crazed monsters are at work too and kidnapped one of the siblings (who was dressed as a candy corn). Even though it's straight forward and to the point in terms of game-play and obviously it was made for the Halloween season (guess when I got it); by far this is not a mere seasonal novelty game.
What impressed me the most is the trademark game element of costumes becoming real and battle ready and the entire innovative, creative roster of ideas that was poured in. In order for a bunch of trick or treating kids to be able to stand a fighting chance against real monsters, their homemade Halloween costumes become real too...a cardboard robot becomes a battle machine, unicorns are real and a vampire costumes becomes well. What I like about these costumes is the very much home-made nature of the outfits and they pack a lot of child-like whimsy as a result. Some of the missions are looking for materials to make the costumes.
The child-like, Halloween whimsy extends to the cartoony world. The atmosphere is cartoony and reminds me of some humor/cartoon style from series like the Fairly Odd Parents. The main game contains a neighborhood, mall and countryside village to explore. A bulk of exploration is going trick or treating to houses in each area, being greeted either with candy or a vicious monster at the door. The extra game feature is collecting candy cards depicting really odd but sounds delicious Halloween treats/recipes (that seem like a gamer's challenge to reproduce)
and collecting battle stamps to power up your costumes.
The Steam version recently got a Trading Card clearance and it contains both the main game and the DLC Grubbins on Ice....I wouldn't say its a Halloween novelty but rather a small gem of a game from Double Fine.