Obviously, there are hundreds of snowmobile trails across the United States, so I will only focus on the lower penninsula of Michgan. The most well known area for trails in lower Michigan is in Cadillac, Michigan. There is a huge system of trails in cadillac, with many turn offs from the trails to go off trail, and fun fields to play in. In fact, there are so many turn offs many haven't been touched even though thousands of snowmobiles ride by them daily. There is also a very large straight trail that starts in Kalamazoo and ends in South Haven, it is about a 2 hour ride and is used in the summer as a walking, biking, skating, etc path. Both of these trails cross plenty of roads that you can ride down to find a gas station, or food. When snowmobiling it is often helpful to stop at a restraunt and get a snack to eat, because if the bumpy trails do tend to make people hungry. And you may go a few miles without another chance for food, so if you are feeling hungry it is a great time to pull off and get some food!
The image below is by far my favorite portion of the Cadillac Trails and is very near a popular spot called the Boon Bonfire circle. It is a small circle and whenever you show up, you can often find new cans of beer and other drinks because people like to treat eachother, there is a pile of wood a couple feet from the fire pit and the fire is expected to stay burning at all times.