Everybody's favorite lit-up phallic symbols! They are sure to inspire sexy-time fics, y/y?
Or perhaps you'd rather take the prompt more literally and write about a visit to a lighthouse. Or maybe a fic about a lighthouse keeper? I can see your hero now, lighting the wick and leaning out over the gallery railing, singing
"Candle on the Water" like Helen Reddy in Pete's Dragon. *happy sigh*
More information about National Lighthouse Day
here.
Headers light the way to awesome fics!