Most European cities have an old town/old section and in that area, there's frequently a central market. Helsinki's market is located right in the middle of the old harbor area, right there by the piers full of boats and ferries ready to take tourists on various day trip jaunts.
The dockside market.
Maybe you're looking to put together items for a great picnic? Start with fresh berries and add other foodstuffs.
How about lunch or breakfast? It's never too early to eat salmon, especially when you see and smell the beautiful planks of it being grilled bright and early. Mmmm.
Non-food things like crafts and textiles are also available, drawing thick crowds.
The breakfast of Finnish champions: Coffee, cruller and waffle with chocolate syrup and whipped cream.