Why not a bike? It’s good activity not only to move your body, but also it’s more interesting to see the world from bike-saddle perspective. Rent-a-bike services are not only in big cities, but often in little touristic towns it is also possible to rent a bike.
Unfortunately in Poland we don’t have too many routes for bikes, often cyclists have to ride through normal streets, what sometimes may be dangerous. In big cities it is also a problem, which is caused by the lack of bicycle paths.
However there are few very nice bike routes, for example 76 km route from Warsaw to Runda through Kalwaria Mountain, surroundings of Krakow with Ojcow National Park or a route through Hel Peninsula (43km).