I agree with Caryl, but personally, I like the northern end of the park(north of Totaranui, near Mutton Cove). Less people around as its further from main access points.
Tonga Bay is great for snorkling as its a marine reserve, There are also lots of Native fish in the stream by the campsite.
I also really liked Bark bay. Nice inlet with a waterfall and safe swimming when the tide is in.
Talking about it is making me want to go back there again. I love the place