i dont think it matters where you go up here, its all excellent. there are approx 50 beaches within an hour of whangarei (an hours drive to paihia) & probably another 50 in the far north (taipa, karikari, monganui etc). i wouldn't plan on travelling much in the 8 days as you'll spend more time travelling than enjoying where you are. i spent 2 1/2 weeks at Otamure bay DOC camp just over the hill from whananaki north & can highly recommend the camp even though it doesn't have power.
As for fish shops, Northland pet & aquarium on cameron st next to the police station is the best up here.