Check a foliage map for that late in Bar Harbor. You might want to go there first depending on leaves and weather. You'll be able to find lobster easily but the dockside lobster shacks usually close Columbus Day. We usually stay on I95/295 until Brunswick and then get off on Rt 1 which goes thru towns like Wiscasset and Camden. We've only eaten at Just Barb's in Stockton Springs once but it's still recommended on Chowhound. Great place for lunch right on Rt 1. If you like wine, there are a couple of wineries you can visit. I like the Lincolnville location for Cellar Door Winery. It's only a short detour off Rt 1 just north of Camden. BTW Camden was voted by Yankee Magazine as best place for fall foliage.
The Portland foodie tour is fun. It will end within walking distance of Duckfat which you must visit at least to try their french fries. If you like ice cream, do not miss MDI ice cream on Exchange St. Their only other location in MDI. Go when it's slow for some free tastes before you decide on flavors for the sampler.
If you like short, easy hikes, in addition to the walk to Owls Head Light near Rockland, there are some nice walks in the Harpswells area (east of Brunswick). The Giant Steps are like a mini version of Ogunquit's Marginal Way. Hikes are on conservation properties. There's one private beach you might not be able to access in a rental car - it's the one that warns about rough road and ground clearance. (We were in a 4x4 pickup so didn't have a problem.) The maps were in a summer tourist newspaper this year.

A few things we enjoyed in Bar Harbor were hiking around Jordan Pond followed by lunch at the Jordan Pond House, kayaking with Aquaterra Adventures, going to the "quiet" side of MDI to hike on Ship Harbor Trail and Wonderland Trail and to see Bass Harbor lighthouse, breakfast at Two Cats, and eating lobster anywhere there (we even enjoyed the Fish House Grill which many label as touristy, but hey, we were tourists and they have a prime spot there on the water with a great view) and eating ice cream from Ben & Bill's. We thought MDI ice cream was okay, but preferred Ben & Bill's. We also enjoyed a whale watching tour, but I'm not sure how that would be in October. Lots to do there!

If you arrive early in the day, take a short trip to Cape Elizabeth to see Portland Headlight. It would not be far from the airport and then stop at the Old Port in Portland for ice cream, etc. on your way North.

I agree with benitakaren about stopping at Portland Headlight. It's a short drive from the airport and affords some great photo ops.