You can't possibly go to the island and not hit a fudge shop. I guess the warning here is not to set great expectations. We thought we were going to be walking from store to store, sampling fudge and watching it being made. Of the 6 or so stores we hit, there was only one "live" creation going on. We must have been there at a bad time. Also, only one of the stores offered samples - in a few of the others, they had a few behind the counter that they would pull out if you asked. Note that there are many "repeats" of stores along main street (a couple Joanns, Mays, Rybas, etc.) In the end, I think we paid $16 for 1.5 pounds (you know - buy one pound, get a half pound free...), and we did get to get about 6 different flavors in that pound and a half.
Unique Suggestions: Enjoy it! It still tastes awesome!
As much as I hate to admit it, most of Main Street is one big tourist trap. Regrettably, many tourists don't even make it out of town, which is where everything worth seeing is. Many of the shops on Main St. are filled to the brim with items from China that aren't even remotely related to Mackinac Island. If you know where to go, though, there are some pretty nifty little shops. Great Turtle Toys, the Island Bookstore, Nephew's, Shirt Tales and Monkey Business are all worth your while.
On a different note, the restaurants along the main drag are all excellent. I have comments on a few of them in my 'restaurants' section.
Unique Suggestions: See above ^^
Fun Alternatives: The less-traveled but no less beautiful street is Market Street, which parallels Main Street. There are probably eight roads that will lead you a block inland to Market Street. It's a lot less busy and a lot more Mackinac-- lined with lilac trees, quaint little shops, and historical buildings. The post office is located on this street, as well as many of the big old cottages that the island is so famous for.
Every tourist on Mackinac Island HAS to visit the Grand Hotel. This hotel is the most famous one on the island and affordable only to the rich and famous (and lucky), but for $10 anyone can go on the porch and into the lobby to look around. Suites in this hotel run approximately $1500 per night, depending on which one you are staying in.
Fun Alternatives: If you don't want to pay the $10 to walk on the porch and the lobby, scaling the grounds (in my opinion) is just as fun. You can always buy a post card!