When you walk around in Gamla stan and pass by restaurant Österlånggatan 17 (name=address), check out the funny signs they put around their outdoorseatingspace. They change it almost daily and it will for sure put a smile on your face. And while being there, you should have a drink outside in the middle of the old town. Or also inside in winter. You might have to wait a few minutes, but the friendly staff will try to find you a table.
Update June 2021: They unfortunately don't have the funny signs anymore, but the food is still delicious!