Rangiroa is an atoll, and it's all about coral reefs rather than sandy beaches. Cyclones washed away much of the sand, but the views are still very beautiful and the lagoon colors are simply stunning. You can sit all day under the coconut trees on the coral beaches and feel you are in paradise if you just bring a towel or blanket. The lagoon water is warm and great for swimming.
If you're looking for snorkeling or SCUBA diving, you've come to the right place. Avatoru has two passes that offer drift diving and great snorkeling in the lagoon right off any beach. (Bring water shoes as you'll be walking over coral pieces and rocks.)
Locals advised me that you can swim in the lagoon with no fear of the many sharks and rays. However, they emphasized more than once that it is very dangerous to swim on the ocean side. (The exception is if you're an experienced SCUBA diver and you're doing a supervised drift dive in the pass.) I was in Avatoru for three weeks, snorkeled off the beach about every other day, and saw sharks and rays every time along with the expected schools of smaller colorful tropical fish.
For a day trip, Lagon Bleu is spectacular, and you will see schools of baby sharks there. It takes about an hour by boat from Avatoru. Ask your pension owner which boat operators are running trips. Lagon Vert is smaller but equally pretty; there is a fish park where you can swim with large tame sharks and rays.