Poipu Beach is a great place to just hang out or take the family to the beach. There's a secluded little lagoon area that's great for little kids to swim, you can snorkel or just lay out.
Lawaii Beach is a great snorkeling beach but not a great beach to just hang out at.
We wanted to find a beach that was a little bit off the beaten path and found Salt Pond Beach on the west shore. This was a good beach to lay out, swim, and take a nap under the coconut trees. There isn't good snorkeling. I felt like we were the only tourists, as it was mostly locals at the beach.
Queen's Bath is actually a swimming hole at the rocky shore of the ocean. It's a fun walk in and is a unique experience. Be sure to go at low tide.
Ke'e Beach is the farthest you can go on the north side of the island and is a spectacular setting. This also seemed like a good snorkeling beach, but we had forgotten our gear.
Shipwreck Beach (at the Hyatt and near the Embassy) is NOT a good beach to swim or snorkel. It is a nice place to lay out and is one of the spots to boogie board. The four of us tried out boogie boarding at this beach and it was pretty thrilling. I got pummeled into the sand more than once and we all left with cuts, bruises, and sand burns (as weird as this sounds, we also had fun). I definitely wouldn't send my kids out there!