I'm not a massive amount of help ( I am only ever in Portland to shop - we drive down from New Brunswick, Canada). However I know there are a few restaurants in Portland that have been featured on different Food Channel shows - we didnt try any of them as we were only there a day,but my husband has told me we will be next time. I'm told that the port area is beautiful, and you might get to see a cruise ship in town, Portland recently started attracting the ships. I still find it fun to see the ships in the Harbour in Saint John NB, even though we have 2-3 a week in throughout the summer months.
The weather will likely be cool, but the leaves should be gorgeous that time of year.
I hope you have a great trip!