Carey, I'm so glad I saw this post
I lived in Portland for 18 years, until I moved down here to Atlanta, 10 years ago. I so miss the coast and the ocean.
The Lighthouse you MUST see is called...Portland Head Light. It is in Cape Elizabeth..which is 2 towns over from where you will be docking. If you can rent a car, that might be ther better way to go. PHL was commissioned by George Washington and is one of the oldest Lighthouses in the US. They have a little giftshop there and it gets loads of visitors.
You will see several other Lighthouse on your way into the harbor. There is Spring Light which is in South Portland and is on a little rock walkway from the beach there, wonderful views of inner harbor there. Its just a really small light, but its quaint, I took my partner there when went to viist Maine 2 summers ago.
If you want more info on Portland, just email me I'd be happy to provide you with other info