One of the greates picnic spots outside the city center is the small and yet relatively famous beach Pikkukoski, on the riverside of the Vantaa. Climb up to the cliffs to enjoy beautiful view over the river(while you still can — they are going to build some big appartment blocks in the area:() or lie down in the small beach. On warm days the beach is packed, but don’t worry there is always room for one or two more.