That weekend's a great weekend to visit Petoskey, but most of the Mackinac Island stuff will be winding down. If you're looking to stick around Petoskey/Charlevoix/Harbor Springs, I'd say go for it...it won't be a busy weekend so you'll be able to enjoy shopping, restaurants, etc. The fall colors will be winding down, but if it's a nice weekend weather-wise, there'll be plenty of outdoor activities.
If, on the other hand, you're looking to hit Mackinac Island, Mackinaw City and do the attractions and real touristy stuff then it might be a little too late in the year. We usually make a fall trip to the island and it seems like most things start shutting down in mid-October.
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