Let me know if you find anything cool, I'll be there for 5 days in July. There are a heck of a lot of galleries etc. to see there and it seems a place called fotografiska Gallery & Bar is very popular.
Other things on the list are Drottningholm Palace, Moderna Museet (Modern Art Gallery), National Museum, Millesgarden, The Ericsson Globe, Worlds Biggest Ikea (about 20 - 30 minutes out of stockholm?), Sergels Torg and a bunch of other things.
Kulturhuset is a a great place It has lots of art and design related things to see and do, as well as a design library and a huge community creative area with printers, computers, sewing machines, studios, and lots more. http://en.kulturhuset.stockholm.se/
@Letsmakeart - yep there were heaps of galleries, large and small. Fotografiska was worthwhile with some revolving photographic exhibitions (and great cafe!). The Haiti photographs were particularly moving.