Unless you count the Horcruxes... but they are vital to the plot since they are the whole reason Voldemort managed to survive.
Also, can't they think of a better name than MacGuffin? I will never willingly use that word in a spoken conversation for fear of sounding like a tosser.
I'd say the Horcruxes don't qualify because you learn what a Horcrux is, how it's made, you see them, and they're seen, defined, and destroyed. MacGuffin's are important to the story because they're important to the story, but they're not directly related other than to get someone from point a to b, where the real story occurs.