The first, and probably the best known is Bronner's in Frankenmuth. This place is immense! They have thousands of ornaments, hundreds of wreaths and trees and who knows how many other decorations, books and toys. They also have a wonderful selection of Hummels and Christmas village houses and accessories. You can easily spend several hours here, so I suggest making Frankenmuth a day trip. Hit up Bronner's and then head over to either Zehnder's or Bavarian Inn for yummy chicken dinners. From either restaurant, you can easily walk the downtown area and check out some of the other local stores. Check the Frankenmuth website for deals and special events, like Oktoberfest and Snowfest.
The other Christmas shop is Always Christmas in Canterbury Village. Located in Lake Orion, Canterbury Village has a restaurant, an ice cream shop and several smaller shops that surround Always Christmas. Always Christmas is smaller than Bronner's, but still large, with two stories of trees, ornaments, and decorations.
If you need a little help getting in the holiday mood (or just want to cool off on a hot summer day), I suggest checking out one of these great places. It's impossible to leave either one without humming a carol!