Bring shed loads of cash. Was there in early July for 5 days with some friends. Great city, with amazing roof top bars around Stadsgarden.
Things worth doing/seeing/experiencing
Food - Pickled herring in a myriad of sauces. The Pelikan Restaurant isn't too bad.
Markets - Östermalm Food Hall for the best produce available
Beer - Local brews are superb, some of my favourites included All Hale to Nothing & Pickla Pils
Architecture - Virtually every building is pretty awesome
Museum - Historiska or the Mordena Museet.
Live Music - Debaser