Steff D.
Yelp
I'm surprised that Yelp has Fort Williams and Portland Headlight as two separate things. As a local, I don't see them that way; when you're at one, you're basically at the other. Unless you're intentionally trying to ignore that giant white spire that's flashing light out to the sea and blasting it's fog horn every minute.
Fort Williams is a lovely, charming place to bring your family and dog (who doesn't love a swing set or a good trail to meander on?), your friends to skip rocks or for a dip in the ocean (the "beaches" are quite small especially when the tide is in, but definitely manageable if you can brave our Atlantic temps), or your significant other for a picnic (SO. MANY. LUCOUS. GREEN. ROLLING. HILLS. What is this, Ireland?; there are picnic tables and grills, too). If you're the exploring type, you can climb in, around, and on some unfortunately graffitied forts... you can't, however, get inside the Goddard Mansion anymore (I mean, you could try, but there's a giant fence warning against such chavvy behavior).
Also - as others have mentioned - it's free and there's loads of parking (two things that can be hard to find in Ye Olde Greater Portlandtown). And, again, it's a positively breathtaking spot to take folks who are generally landlocked or big city dwellers to; absolutely worth coming to at least once in your life (and, if you're fortunate enough to be a local, then as many times as you possibly can when it isn't tourist season, as we all know it can get obscenely busy here June to August).