Have been to both lots of times, but not in the last few years. Here were some of our favorite things:
Mackinac Island
Rent a self-drive carraige...how often or where do you EVER get to do this?? They are really easy to drive and the horses KNOW the way!
Go to the Grand Hotel for lunch, dinner, anything...it is beautiful. I didn't like staying there too much but it's is a really great place to visit. They may charge you to just look around, but it should be free if you are eating.
There is also a pub on Main St. that has great food (sorry I forgot the name).
The Iriquois Hotel has a great restaurant with lake views.
Take tea at the fort.
Tons of historic stuff....the fort, houses, etc. if this interests you. I love history/museums and DH thinks its BORING.
French Trader bar if you are there at night for music!
Buy fudge...EVERYONE does!!! That's why the islanders call tourists fudgies!!
Shop..shop..shop....bike...bike...bike...eat...eat...eat....hike...hike...hike!
Harbor Springs
Lots of neat shops and restaurants. One of our favorite restaurants was called The New York (not sure if it is still there).
Boat watch...there are some truly amazing boats that pull into Harbor Springs.
I liked staying on Mackinac Island much more than Harbor Springs. At night, the Island is truly magical and romantic and there is more to do. Perhaps you can change your reservations around??? There are a ton of hotels and B&B's on the Island and some are pretty reasonable. The Grand Hotel is expensive ($300+ a night) but there are other hotels that run around $150 or less. We stayed at Mission Point one year and it was great.
Both places have tons of festivals, too. Art festivals, fish festivals, lilac festivals....so hopefully you'll hit a weekend with this added attraction.
Either place is fun and relaxing...ENJOY!!