This will be my second time back to the islands and the only advice I can give is what we enjoyed the first time we were there.
We did the stingray excursion and loved it, I have never did the one in Grand Cayman so I can't compare .
We loved the 4x4 in Bora Bora and the Jet ski (I forget what island it was but it was fun!) We also rented scooters in Moorea and that was so much fun we are doing it again this time.'
I remember doing several snorkel trips but I can't remember which islands or which excursion!
In Papeete, we visited the Paul Gauguin house which killed some time ;)