If you go to Boulders Beach and all you do is follow the mob of tourists to the penguin-viewing decks, you're wasting your day (and your money)! You'll get some good pictures of napping penguins there, but the real fun begins when you go to the secluded little beach off to the side where you can actually swim with the penguins and take close-ups while they're lazing about on the boulders. It's a quiet little beach with gentle waves that's safe for children as well, and the color of the water is simply amazing.
Tip: make sure you go at low tide if you want to enjoy the beach. Wear your swimsuit and pack a towel, sunscreen, water, and some snacks.