I usually have nothing to do.

It means: Do it when you have nothing to do. This is a code word on the Internet.

Connect one word of each fruit with the last cake, and the result is ________: apple, tomato, persimmon cake, which is a homonym for usually nothing to do.

Apple, watermelon, strawberry and tomato cake literally means fruit cake with apples, watermelon, strawberries and tomatoes.