The market here is open pretty much everyday (except in the winter when they are limited to fewer…read moreshops indoor) and is one of the tourist spot to stop by, either for a regular city tour or while waiting for a ferry to Suomenlinna.
The shops sell things ranging from crafts and souvenir to fresh produces like berries and veggies and cafe food. It's a nice quick stroll to see Helsinki life :) (though many tourists come here more than locals I think).
Pluses are that there's no entrance fee but, as a big foodie fan, I wish they have more selection for edible food to try out!