Hello, I am a spanish guy (29) living in Hamburg and I would like to visit Stockholm between the 27th of april and the 2nd of May. I would like to meet people there, travelling alone is (sometimes) boring. So, I would be grateful to contact people who could give some advice about what to do, what to see and so on.
I am not going to go to Stockholm with you, but be sure to visit the Vasa-Museum and the Old Towm (Gamla Stan).
I don't know if you booked your flight yet, but take a good look at which airport you will be flying on. Some airlines (incl. Ryanair) use airports that are up to two hours travel away from Stockholm. If possible take a flight to Stockholm-Arlanda, the nearest, that also has a good connection to the city by train.
Yes, Gamla Stan is not to be missed and the area around Slussen and down that line is pretty nice.
Within Stockholm, 'must see' places are the Prins Eugens Waldermarsudde art gallery and house, Lidingo and Raoul Wallenberg's statue at the Lidingo centrum, the tiny courtyard walled garden behind Engelsbrektkyra near the Tekniska Hogskolan. There is a nice italian restaurant, Ciao Ciao, serving great pastas for very reasonable prices near Hogskolan as well. These were some of my favourites...
Also, if the weather is not too cold, do go to the archipelago for a weekend. It is beautiful and fun!