I can heartily recommend Marcus at Air Ace at Groningen Eelde airport for refuelling and handling. They can also pick up a delicious packed lunch for you if you're tight on time, and have excellent coffee. If you choose to stay in Groningen, it's a quiet, pretty academic town with lots of pleasant bars and eateries. It's also just south of the main military operations areas which makes navigating the restricted areas somewhat easier. It's about 10 minutes by air from the German border.