Clementines always seem to pop up around this time of year where I live... I've always loved them. I'm a fan of citrus fruits, but not of peeling them. Clementines simplify things greatly, and they taste great!
On that note, I also love pomegranates, and they're reasonably priced at the moment. They're not easy to peel at all, but I find the submerged-in-water technique is the least messy (as compared to the slice-and-whack...).
Of course, while it's not a seasonal "product", so to speak, I drink a lot more cocoa once the weather cools off. Currently I am sitting here with a cup of mint cocoa with tiny marshmallows.
So many to choose from...but it'd have to be either pumpkin pie or eggnog.
Yeah, you can make those in other seasons, but they don't fit nearly as well.
Oh...and peppermint tea, or hot cocoa with a candy cane as a stir stick. xD