On the main archway of the huge sand castle, one of Patrick's socks sits on top of the right pillar. By knocking down a stack of floating stone tikis, SpongeBob can climb to the top of the archway and collect the sock.
There are many umbrellas around the level. Hitting them with the bubble wand causes them to spin for a moment. By timing a jump on the umbrella beside the castle's archway, SpongeBob can gain enough upwards momentum to reach the sock, without climbing the tikis. This is useful for Any% No CBG.