Not Viking related but if in Stockholm you should (As Reavor mentioned) absolutely go to....
Welcome to one of the most visited museums in Scandinavia and explore the world's only preserved 17th century ship. The ship Vasa capsized and sank in 1628. After 333 years on the sea bed she was salvaged and the voyage could continue.
www.vasamuseet.se
Skansen is the world’s oldest open-air museum, showcasing the whole of Sweden with houses and farmsteads from every part of the country.
www.skansen.se
Birka is cool but wasted if you do not go on the guided tour since there aren't any ruins per say, mostly just grave hills and left over earthworks. The guides are engaging and fun though and fill in all the holes. Just the trip over can be fun on its own if the weather is nice as well. It is 2 hours one way though if you take the one that goes from Stockholm so bring a friend.
Would you want to explore outside Stockholm as well? Are you renting a car or sticking with public transports?
As a good to know, there is a travel ban currently on non schengen/eu citizens. This is in effect until June 15th. It is not impossible this could be extended by an additional 30 days (They've done this once already).