Flensburg or Kiel

Regensburg because it's about half way.

OK, Hamburg is a big city, but I think there is a lot to see. Kiel has been destroyed almost completely during WWII and I did not find it interesting. I have no opinion about Flensburg. BTW, how do you plan to travel from Denmark to Slovenia? Aren't there other towns that you could visit? Lubeck, Travemunde, Bremen?

I have planed to stop in Wuerzburg. So i have one day between Arhus and Wuerzburg.

Hamburg then. http://www.miniatur-wunderland.com/ is well worth a visit.

Can you explain us the following:

Firstly, how you travel around ? With a car or by rail ?
and
one day between Arhus and Würzburg means that you depart in Aarhus in the morning
and arrive in Würzburg the same day in the evening or the next day ?
are you planning to sleep in the city you choose to visit ?

Kiel or Flensburg?
Flensburg.
Nordermarkt, das Städtische Museum, Schifffartmuseum etc. Also nice panorama from the museummountain.
Besides that it's a 'soft' way to enter a new country: You'll for sure meet many Danes in Flensburg for some shopping (beer) and sightseeing.

If you like a touch of the Netherlands (canals and facades) means only a very small detour to Glückstadt,just northwest of Hamburg.

Another village with a touch of the Netherlands is Friedrichstadt on the river Eider. Another interesting place is Schleswig.