The clubhouse is a small, wooden shack sitting on top of a large vine protruding from the sand. There are many other thinner vines around it. It also lacks a lot of space, due to SpongeBob and Patrick being stuck in it for 3 days. The inside of the clubhouse is probably small.
The clubhouse is first seen with Patrick and SpongeBob sitting inside of it. Squidward goes by it riding his bike on his way to work and SpongeBob and Patrick start to make fun of him using their "secret code," which in reality is the overuse of the letter W. Squidward becomes aggravated and tries to get into the clubhouse, even though SpongeBob and Patrick claim it is not big enough for all three of them to fit in. Squidward manages to fit in very tightly, so SpongeBob and Patrick give him the new member initiation, which is just them clapping and yelling "welcome Squidward" over and over again. Squidward gets annoyed and decides to leave, by climbing down one of many vines. Squidward's entire body doesn't quite fit through, and he ends up dragging the clubhouse with him. When Squidward lets go, the entire clubhouse is sent flying towards the Kelp Forest where it is broken due to the impact on the ground.
SpongeBob claims it's so small that he and Patrick had been stuck there for three days.
Despite the fact it is located next to Patrick's house, neither the clubhouse nor the vines are ever seen again.