They made war with each other. There used to be an union betwen the three scandinavian countries called the Kalmar union it was lead by Denmark and Sweden was quiet unhappy with it. About the time that the age of sails starts Sweden leaves the union. This leads to lots of wars. Sweden is at that time weaker than Denmark-Norway, so they have a number of wars where they try to expand to better defend themself, while Denmark starts a number of wars where they try to force Sweden back into the union. This see the raise of the Swedish empire that dominates the countries around the baltic sea. At this point Sweden and Russia also starts fighting some wars against each other as Russia wants acces to the baltic sea and Sweden wants to dominate it.

It ended out in the Great Northen War 1700-1721 where Denmark and Russia teamed up to beat Sweden once and for all. A lot more countries took part in the conflict though. Some for a long and other for a small amount of time. The conclusion was that Denmark and Russia won. The Swedish empire was more or less torn appart and their dominance of the baltics ended.

There would still be a number of wars fought in the north over the next hundred years, but the northen empires was at this point more or less destroyed and they often tried to stay neutral in European affairs.

So while all this war was going on Sweden and Denmark also had a bit time to go get a few colonies. Most importently Denmark reestablished control of Greenland, but both countries got a number of minor colonies around the world. They also both made trading companies and at some point both Sweden and Denmark smugled more tea to britain than britain got through its own fleet.

So basicly a small bit of colonisation, a bit of trading and then just lot of war!