Nuuksio national park is one of the must-places to visit near Helsinki, so if you have scheduled an extra day to your visit, you would do well to head towards the Finnish wilderness. There are several nice trails marked through the forest, which all have some outdoor barbeque-spots for those wishing to have a snack out in nature (highly recommended). There are also lakes in Nuuksio for those wishing to have an even more immersive experience.
More information on Nuuksio can be found at http://www.nationalparks.fi/nuuksionp
If you wish to have a knowledgeable and good guide to Nuuksio, you can find one at https://www.finnishfriend.com/