The June, 1950 issue of Popular Mechanics featured a collection of buildable inventions to take to the beach, including this floating playpen, with four auto inner-tubes to prevent tipping. I love the playpen especially, because it has the look of the sort of device that produces incredibly sweet nostalgic memories in 99 percent of its users, and terrible memories of drowning tragedies (or near-tragedies) in the remaining one percent.