North Frisian IslandsSchleswig-Holstein in northern Germany, separated from the mainland by the Wadden Sea.
HalligenBesides the major islands, there are also the Halligen, smaller islands with most of the activity limited to one or several Warften, man-made hills which are built to protect the inhabitants in the event of a storm flood and which are usually the only thing not to be flooded half a dozen times a year. Their name is sometimes used with "Hallig" before their proper name (e.g. "Hallig Hooge")
GrödeAt nine inhabitants, this is the smallest community by number of inhabitants to elect its own mayor. Naturally there is no such thing as a "town council" as all inhabitants over the age of 18 get to weigh in on matters usually decided by mayor and town council
This is largely a rural area with tourism as its main (and sometimes only) economic activity of note, so the term "city" is a stretch even for the ones listed here
- The Halligen, small islands and islets that are almost completely submerged during high floods save some Warften (artificial hills with most of the settlements on them). The biggest and most well known of them is Hallig Hooge (pronounced with a long "o").
The islands, and the water around them, make up the Schleswig-Holstein Wadden Sea National Park (Nationalpark Schleswig-Holsteinisches Wattenmeer).
By airWesterland on Sylt has an airport with some seasonal connections to Southern German destinations. However, unless you are headed to Sylt first and foremost, its location in the Northwestern "corner" of this area makes it an awkward access point, despite its railway connection to the mainland.
Besides that a number of islands, for example Föhr and Heligoland have small airfields for private aircraft. For private charter Air Hamburg is one possibility. Hamburg (HAM) airport is the closest with major airline connections. That being said, unless you are already in (Northern) Germany, you will most likely find arriving by air cumbersome and too expensive for hardly any value added.