Even though Vienna is not close to some mountains (as, for example, the city of Steyr, where I was working for three years), there are quite a few possibilities for outdoor sports within the city. The Donauinsel (Danube Island) is especially popular.
I am enjoying both cycling and rollerblading on the Donauinsel. The numerous paths along the so-called "Neue Donau" (literally "New Danube", an artifical side arm of the Danube river for the purpose of flood prevention) are very suitable for these kind of sports.
As shown on the map above, the Donauinsel stretches along a distance of about 22.5 kilometre. It is easily accessible by two underground lines, U6 and U1. From the two underground stations, the island stretches about seven kilometres to the north (red line), and 13 kilometres to the south (green line). The two stations are separated by almost 2.5 kilometres. Naturally, this area is the most visited part. Still, usually it is not too crowded, either.
Apart from the three parallel pathways on the Donauinsel (one each on the shores of the Danube river and the Neue Donau, as well as one in the middle of the island), there are two more paths east of the Neue Donau (one of which is shown by the blue line in the map above). Therefore, there is more than enough room for all the bikers, roller bladers, and (sun) bathers (as it is possible to swim in the Neue Donau as well).
All in all, the Donauinsel offers an excellent opportunity for escaping the city every once in a while – both for Viennese spending their vacations at home, as well as for travellers who have recently returned from a long-term trip, and are currently looking for a new job.