When it came time to purchase a good nativity set, I went with one that had a ton of pieces available so I could build it over time. (Frankly there are enough pieces in the set to actually build all of Bethlehem - and most of the rest of the Bible as well. Both testaments.) The biggest factor was you can purchase a Mary & Joseph where Mary is still pregnant and riding on a donkey. As part of our countdown until Christmas the kids get to move Mary & Joseph a little closer to the empty manger each day... they start in our garage and work their way into the house, getting a little closer each time. On Christmas morning, we'll switch to a 'new' Mary and add little Jesus to the manger.
It's a fun way for the kids to know we're getting closer to the holiday while also providing a visual reminder of the 'reason for the season'. So here's the Holy Family hanging out on the pantry shelf in our garage and then on a shelf in our living room. They are currently making their way across our refrigerator:)