You're right. They're so damned cute that it's ridiculous. Grape-sized watermelons. Not exactly. These little Curcurbits taste like cucumber with a bit of watermelon rind and a squeeze of lemon juice. The crunchy little critters are tasty, no doubt.
I first read about them perhaps four years ago and got to play with them for the first time last summer. And I have more in hand right now. This native of Mexico and Central America, Melothria scabra, besides being known as a cucamelon also goes by several other monikers such as pepquiño, mouse melon, Mexican sour gherkin, and sandía de ratón in Spanish. It's still too early to know what name will catch on here in the US.
Although the cucamelon tastes like a cucumber-melon cross, it is botanically neither and won't cross with either. The vines with deep green ivy-shaped leaves are beautiful and lend themselves to trellising. If I had more room in my garden, I'd grow these prolific little fruits. At 75 days to maturity, they are well suited to our growing season here in Virginia, seeded out May 15 and fruiting the first of August, or even faster if transplanted from the greenhouse.
I don't have a whole lot to play with right now, but my experience over the last two summers is that they store very well under refrigeration. Right now, I'm just eating them out of hand—the whole fruit is edible just like a grape—and putting them in salads along with heirloom cherry tomatoes. I'm looking forward to pickling some when I have more quantity.