Stockholm is a town surrounded by water, and even if gives a beautify scenery during day time, it becomes even more beautiful at night time. So why don't take a walk at the waterside, I sure you will get a good a peaceful moment.
Dress Code: Warm clothes are never wrong during late Stockholm nights.
Can you believe I have no guildebook ar whatever in hand when I was there, at night alone walking on the street watching local shops closing as early as 7pm. I don't understand Swedish but that's was fun just walking nowhere no direction and nobody knows who am I, yeah that's my Stockholm at night, later I bought a can of local made mineral water and a burger-like Swedish supper walking back to the hostel.