I usually read a lot about the city in advance and make a list of sights I want to see. And most time I use to stay at least one night. So I arrive the first day and use to walk around the city. In most of the cities (except Berlin) most of the sights are near to the city centre. So most parts of my list are done without any planning. For the next day I only have few sights left. Mostly things apart the centre or indoor activities like museums.