Presque Isle in the wintertime: the circle drive is closed off to vehicle traffic and becomes a beautifully groomed X-country ski trail. Snowshoeing is ideal on the interior paths. It's nice to take a glimpse of the ice formations on the Black Rocks, and to look out for the island deer to see how they are coping through the winter.
I've already mentioned Marquette Mountain, and it's a great place to ski/snowboard (however, I only have ski experience). There are beginner to advanced runs. When you need a rest, the Chalet is a nice place to hang out and relax. It's right at the bottom of The Rocket (Black), and it's fun to watch the people go down it.
The staff is super friendly, and they aren't over priced. However, the food at the snack bar area is overpriced, and doesn't taste that great. But that's pretty typical.
Overall, I would recommend visiting Marquette Mountain if you are going to be in the area. I haven't been skiing anywhere else in the U.P., but I'ave always enjoyed Marquette Mountain.
Equipment: Definitely bring your own ski boots if you have them. I've always rented their skis, and I've had no problems with them.