Mackinac is a must do in my book. It's so pretty, and you can take a horse drawn carriage tour, visit the Grand, tour the fort, eat along the water, and get fudge. The scenery is gorgeous. It can get crowded though.

It is a lot of driving for one week, but doable if you don't mind that. We drive to Michigan almost every summer (from FL now, but formerly from DE, and before that from NY, and before that from MD) and often make the trip up and back in a week, using the weekends on either end (i.e., Friday of the first week through Sunday of the second). The scenery is pretty. I would suggest heading north first, then dropping down to visit your friends in South Haven for a day or two, to shorten the drive home. I think it takes about three hours to get down to South Haven from Traverse City, and then the drive back to Richmond would be a little shorter from South Haven.

South Haven itself is a beach town - that's the biggest attraction. There are some nice small wineries within easy driving distance if you want to do more than watch the waves.

In August you'll get fresh blueberries and, depending on how late in the month you're there, fresh local Red Haven peaches - the best.

There is another thread about South Haven and the surrounding areas. The thread was a few years old but someone revived it a few months ago. All the information is still pertinent.

Agree with volcanogirl that the Island should be at the top of your list. Even if you just make it a day trip from Mackinaw City, it's worth the time. And, don't forget the Fudge!
DW and I really enjoy Traverse City area as well, wineries on the peninsula and great parks. Other than the beach scene don't know about South Haven.

We stayed at the Cloghaun B&B on Mackinac Island, and my parents stayed at the Metivier. Both were lovely. We went horseback riding, browsed the shops, and sampled LOTS of fudge! We were there in August, and although it certainly wasn't deserted, it wasn't too horribly crowded.

We stayed at The Lilac Tree and loved it. Great location close to everything. There are no cars on the island, so everything is done by horse and carriage. It's charming. If you go, we really enjoyed eating at the Pink Pony. Yankee Rebel Tavern is good too. There are fudge shops everywhere. They actually pump fudge fumes out onto the street so it smells amazing.

One week? Lotta driving.
Personally, I'd drop Mackinaw Island, unless it looks so attractive to you after Googling it that you must see it.

Assuming you come up thru the Detroit area, it's pretty much a wash, timewise, whether you take I-94 west to South Haven or I-75 north to Mackinaw. Maybe an hour less to South Haven. But if you do all 3, I'd do South Haven first, then Traverse, and if interest wanes at that point, head back to Richmond on I-75