What do you like? Mountains, flat only, lakes, sea side, sight seeing, hiking, cycling.
You have the north sea and baltic sea in the north. The north sea is very different from the English sea side, very flat, dunes, on the mainland quite boring but has lovely islands if you like watersport. Weather is similar to England so in May you won't be able to go into the sea though.
Baltic sea is calmer, more towns to visit. No surfing as it has less waves. Hamburg is near.
You have lots of mid-range mountain ranges, Rhön and Harz are good areas to go to if you like hiking but don't want Alps. Similar is the Frankonia Switzerland, more hilly, absolute lovely and north of Nuremberg. Public transport is spotty though, you need a car. We hiked this year several times, there are things like Go Ape, climbing, canoeing (River Pegnitz) water parks.
We found the Black Forest fairly boring this year.
The Bavarian Alps or the lakes like Starnberg is supposedly good, I haven't been there apart from the odd day so can't comment but I hear it from people on mn all the time.
How about Berlin? Age wise they would find lots of interesting museums.
We driver from SE London/Kent Border and stay overnight most times. Nuremberg would be a full day only driving. More accessible would be the border region to France (Palatine), Belgium/Luxembourg or Netherlands. If you just go for one week I would look into flying and then hire a car.