By feet, bike, car or with the “Öffentlichen”
Cologne has an extensive network of public transport. With a public transport ticket you may use the different kinds of vehicles. You could combine S-Bahn and U-Bahn or take the bus, you can use all 3 to explore Cologne. If it takes too long for you, of course you could take one of the many cabs in Cologne. Or you rent a bike and enjoy the sunny side of Cologne.
Between the stations you could have a look at the sights of Cologne, like this you get to know the city better. You could take part in one of the car-sharing programs Cologne has on offer, if you want to explore the city and its sights by car. Or maybe you want to take one of Cologne's ferries to see the city from the water.