I've visited Stockholm with three friends, and we liked a lot the atmosphere of this city. I would say that between the cities I've visited in Scandinavia (Copenhagen, Stockholm and Oslo) this is the one I've liked most.
The city is somewhat big to walk by foot, so definitely bring all your energies, you'll need it. And visit the great looking caravel boat called Vasa! It's really worthy!
Insider Tip:Visit the Vasa boat museum! That big caravel is pleasant to the eyes, looks great! I recommend also taking the Free Walking Tours, there are at least two: one in the morning (10 a.m.) and another in the evening (19 p.m.) so ask the hostel staff about them.