A military playground in the part of Rijeka called Krimeja, which used to be a space used by the army, has now been transformed into two handball or mini football fields and a basketball court. The athletic track rounds it up and makes this THE place if you want to stay in a good shape even when you are not in your hometown. There is also a section for a long jump, a shot put part, and a part for street workout where you can stretch yourself.
The best time to train is in the morning, because it is not crowded (in the evening you will probably have to be more careful about other racers and players.)
If you are in Rijeka for a long time, you can inquire and join one of the running clubs and then use the dressing rooms with showers that are just near the track.
The bus that goes up to the playground from the city center is number 2. If you take a taxi that should not cost you more than 3 euros per person. This playground is open for the public and is free.