I'd recommend Hotel Wi Stwosz at 28. Mikolajska Street. Basic, simple but clean, comfortable with a nice breakfast (cereal, meat, cheese, bread, salad, tea and coffee) and very close to the main square (Rynek Glowny) and the Old Town plus a short walk to Planty and a nice stroll down to Wawel and the castle.
We've stayed there each time we've been to Krakow (4 times now) and have not been disappointed.