Nuuksio National Park
The easiest way to get is by car. By public transport there are no direct connections to Nuuksio from Helsinki and getting there will require a transfer. Take the train to Espoo station (commuter trains S, U, E and L) and then take the bus 85 or take train to Leppävaara (commuter trains A, E, U, S, L, Y) and take the bus 28 to Siikaniemi.
Bus services are somewhat infrequent so check the schedules before leaving.
The only way to get around is by hiking.
A number of well-signposted trails are available, while hardcore hikers can head out into the bush and stay at one of the many campsites. An information cabin near the main entrance (as well as the Helsinki tourist info) has simple free maps, or invest 10€ in a detailed topographic map of the area.
Rent the sauna Oravankolo for a night.
Eat & Drink
No food or water is available in the park, so bring everything you need. Bring your sausages and grill them at one of the official fire places.
Several wilderness lodges can be rented cheaply, but they're popular so advance bookings are essential. You will also need to bring your own sleeping bags.
Apart from vipers and the very remote chance of meeting a bear, there is little dangerous fauna in the park. If you don't have a compass and orienteering experience stay on the main trails.