Analise's suggestions are right on target and perfect for your 24 hr timeframe window. Harraseekett in Freeport is classic Maine and the location is ideal -- bring your camera!!! Also, a stop into LLBean is obligatory for every visitor to Maine!
She's also right on with the suggestion to enjoy a harbor ferry trip on Casco Bay Lines -- tickets for a round trip island ferry might run you $10 or so and, if the weather's good, it will be your most memorable experience in years. Just can't beat it. Also, when that's done, the ferry terminal is right in the very heart of the old port with great food and taverns at your beck and call.
Lastly, I'll add that I live in Portland ME! Enjoy your terribly short time here -- next time plan to stay a while!