Stay away from the Red Light District!
Especially at night-time. In Leeuwarden, there are gangs of foreigners waiting to pounce, start fights and relieve you of your cash. If you see a large group ahead of yuou, turn around and leave sharpish. I'm told it's becoming a problem in the city. It tends to happen, as one group approaches the unsuspecting, then other groups join in, starting a fight with the first, with the unsuspecting caught up in the middle. Please avoid, one of the guys working in the hotel had a narrow escape there, and he lives in Leeuwarden.
This must be the only downside of the whole city.
The Wadden Sea is beautiful but full of dangers. Do never go mud walking or sailing here with a local person!