Though I think the Keelung night market is a compelling enough reason to visit Keelung on its own (the fact that it’s so easy to get here from Taipei makes this a no-brainer), you might want to do some other stuff while you’re here. I mean, you’re here already, you may as well.
Or you could just eat. That works too.
But if you want to walk off some of that food, there are a few interesting things around the city that you could check out.