I live in Maine and even in the summer time its the shit. Here's what you could do.
Surf, Scarborough, or York beach.
Cliff/Bridge Jump Frye's Leap or Solmon Falls.
Go to Old Orchard Beach and scope out mad beezys, eat delicious fries.
Go climbing at Maine Rock Gym.
Go to Yorks Wild Animal Kingdom.
Go to Bug Light Park in South Portland (famous downrail there) and you get breathtaking views of Portland.
Go skate portland or longboard.
There's so much more dude its insane, I love Maine, if you want anything in particular ask and I'll tell you!