Travelling to Maine for a Week in June. What are your Must-Dos/Must-Sees?

My partner and I are taking our first vacation EVER as adults and have been wanting to visit Maine for the past five years. We booked a hotel in Portland for a whole week from June 22-29 and are looking forward to travelling a bit up and down the coastline.

A bit about what we are looking for: we are not interested in gimmicks or tourist traps and would prefer to spend our money at cool hole-in-the wall or mom 'n pop places. We love art and history, the ocean, nature, and good food (which is clearly why we chose Maine). We don't have a massive budget, so would prefer to avoid things that are super expensive (as in, we're not interested in fine dining and the like).

I am also gluten free but love seafood, so would appreciate recs for a good GF lobster roll!