When I celebrated my bar mitzvah there was no cake. Today there is no such thing as a bar mitzvah in the United States without a special cake. It can be even more complicated and expensive than a wedding cake because bar-mitzvah cakes are often based on a particular theme.
I was one those kids who had books on them. Before weddings Bar Mitzvahs funerals and anything else where you're actually meant to not be reading my family would frisk me and take the book away. If they didn't find it by this point in the procedure I would be sitting over in that corner completely unnoticed just reading my book.