Kampinos National Park is the biggest green area in the surroundings of Warsaw. It has been designated as a UNESCO biosphere reserve. There are around 16,500 animal species here, including deer (very lovely to watch), wild boar and more. Kampinos is quite easily accessible from the city, so when fighting against the Warsaw smog (on some days the safety limits are exceeded by almost 500%!) you might want to relax somewhere green.
Horse riders enjoy Kampinos as much as dog walkers, mushroom pickers and runners do. People interested in Polish history might want to visit the cemetery in Palmiry, a place where many executed Poles from World War II rest. Kampinos was itself a hiding place for many during that time. People interested in culture and music will definitely want to travel a bit further to Żelazowa Wola, the place where Fryderyk Chopin was born and used to live.
To visit Kampinos for a stroll close to Warsaw, go to bus stop Dziekanów Leśny 01. If you want to rent a bike, check this point in Laski.