Is it realistic to do a quick visit to Helsinki, if I have 5h waiting time between flights? If so, what kind of transport should I take and what is the landmark mostly worth seeing?
From the airport to the city center you can go by:
In both cases, having to wait 5 hours you'll have plenty of time for a quick visit of the city center. From the train station (nice building) the senate square (with the Luteran cathedral and the university), the Esplanade and the market square are all within walking distance. You could even visit the Uspenski cathedral at the beginning of the Katajanokka peninsula.
Here is a short itinerary that covers the very center of the city.