Try to get out in the archipelago on a boat! Also a trip to the fortress island right outside of Helsinki could be good, boats leave from "Kauppatori" at the end of the Esplanadi street. Also just a walk through the city to the seaside in the south and a coffee at Café Carousel or Café Ursula.
All of this of course if the weather is nice... Otherwise the Kamppi shopping center and Stockmann are good for shopping. almost 11 years ago