Cucamelon

These tiny fruits originate in Central America, deliver a taste of watermelon with  a hint of lime.  The trailing vines are rampant and at home in any unheated glasshouse as well as a sunny sheltered spot outdoors.  Despite their exotic origins they are much easier to grow than regular cucumbers and not touched by the usual range of pests.  The plant is actually a perennial and provided the radish like root is dug up and stored much as a dahlia tuber over winter, they will flourish for many years.