Pictures:
Gamla Stan seen from Slussen
The Royal Castle
Although most of the tourists visit Gamla Stan when they come to Stockholm, it's a lovely place to have a wander around. It can get crowed throughout the day but in summer it's also nice to visit Gamla Stan in the evening when most of the tourists are gone. The atmosphere on the cobbled streets is unique and you can find small squares or interesting streets around every corner. You can reach Gamla Stan easily with the metro because it has it's own station and then walk around as much as you like.
There are many attractions in Gamla Stan and one that should be named is the Royal Castle. You can argue if you like the outside of the castle or not but the inside is quite impressive. The changing of the guard is also an event that you shouldn't miss when you are there at the right time. You can have a look at different churches, for example Storkyrkan or Tyska Kyrkan. You don't need to pay to get into Tyska Kyrkan (German Church) and it's worth a visit but not as big as Storkyrkan. A couple of museums are also located in Gamla Stan, like the Nobel Museum.
You can find many shops in the small streets of Gamla Stan, where you can buy typical tourist souvenirs but also clothes or handicrafts. I guess everybody will find a nice place in Gamla Stan and the food there is good too. One of my favourite cafés is still a Swiss pastry shop where you can get tasty pies and cakes. There are also many picture opportunities in Gamla Stan but you aren't allowed to take pictures in every museum.
Have a look here if you want to read more:
http://www.visitstockholm.com