"The Rolling Stones" by Robert A Heinlein, MMPB in Vintage cover

The rollicking adventures of the Stone family on a tour of the Solar System. It all started when the twins, Castor and Pollux Stone, decided that life on the Lunar colony was too dull and decided to buy their own spaceship and go into business for themselves. Their father thought that was a fine idea, except that he and Grandma Hazel bought the spaceship and the whole Stone family were on their way out into the far reaches of the Solar System, with stops on Mars (where the twins got a lesson in the interplanetary economics of bicycles and the adorable little critters called flatcats who, it turned out, bred like rabbits; or, perhaps, Tribbles . . .), out to the asteroids, where Mrs. Stone, an M.D., was needed to treat a dangerous outbreak of disease, and even further out, to Titan and beyond.

Bagged for preservation