I spent a few days in Kassel last year and did some flying around Germany, including the islands of the north coast, which was super.
Route from UK was no problem, apart from no clearance through Duesseldorf class C CTA. Read your Bottlang/Jeppesen though, there are a couple of differences to the UK. For example the standard VFR squawk in Germany changes with altitude and some areas are transponder-mandatory.
If you don't fancy Ostend (which we used for an overnight because we like the seafood) try Kortrijk. Very GA friendly and a nice restaurant.
I used English on the RT throughout.
There are two Paderborn airfields. Haxterberg is grass. Lippstadt is hard & bigger. I have flown over the letter, but not landed, sorry.
Regards