If you're going to the Bay of Islands, I'd avoid Paihia (too touristy, with too many loads of backpacker busses arriving each day), but instead take the ferry across the bay to Russell (or the huge detour to get there by car), which is a much more quiet (rustic, even) and romantic location.
For the Coromandel, don't stay in Coromandel Town, for it's a dump. Whitianga is quite nice. Hahei might be even better as far as location goes (you can walk to Cathedral Cove from there), but the town did appear to be distinctly tiny as I drove through it, so I don't know how much staying power the place has.
I'd also consider Napier or Mount Maunganui. Napier is the nicest city of them all, while Mount Maunganui has a great beach and of course the mount which you can walk over and around for seemingly forever without taking the same path twice.
I think all these places would be hard pressed to remain interesting for a full "few weeks", so maybe you could visit two or three.