Standing above the Momochi district is 234m-tall Fukuoka Tower, Japan's tallest seaside spire, a symbol of the city and mostly hollow (its main purpose is as a broadcast tower). There's an observation deck at 123m and a cafe for soaking up the views, especially at dusk and nighttime. Check the website for the schedule of free 'light up' illuminations held throughout the year.
It's about 20 minutes on foot or 10 minutes by bus north of Nishijin subway station (exit 1).