If you are visiting Aitutaki you MUST go on a lagoon cruise. We went on Bishop's lagoon cruise. The boat we were on had top deck where we took tons of photos and relaxed in the sun.
The tour is all day and it takes you to "Mosquito Island" which is where the "Survivor' cast members were sent once they were voted off. Take LOTS of insect repellent because the mosquitos there are vicious. The island itself is beautiful, but be prepared for the mozzies!
We made snorkel stop somewhere in the middle of the lagoon. We saw big fish, a moray eel and baby clams. The current was a bit strong, but we were still able to snorkel without any problems.
After our snorkel stop we went to Onefoot island where lunch was being prepared for us. It was delicious! Onefoot Island has a post office where you can get your passported stamped, buy / send postcards, and even buy Onefoot Island merchandise.
Onefoot island is where we spent the majority of our afternoon. There were tons of things to do -- walk around the island, snorkel, lay on the beach, or walk to the sandbar. I highly recommend walking out to the sandbar. The water was never more than thigh deep and it was sooo warm and blue. Once we got the sandbar there were only 2 other people on it. It felt like a desert island. The sand is soo white its almost blinding and your'e surrounded by the most crystal blue water. It was truly stunning!