Helsinki in the summer gives you endless light to explore the design district, the iconic white cathedral, and clean-lined architecture by Alvar Aalto. You get warm weather, outdoor cafes, and midnight sun that lets you stay out late taking in the city.
A typical trip runs around $2600 for flights and a place to stay. Spreading that cost out over time makes it easier to manage if you want to lock in your summer dates now.
That 0% window is a hard deadline, not free money. If you still have a balance left when the 18 months are up, standard interest kicks in on what is left over.