A junket can be any trip or journey, but it is used more specifically to refer to a trip taken at the expense of another (such as an employer). You might hear of an actor going on a press junket to promote a movie, for example.
The bands of the regiments were making music at all hours. The greatest folks of England walked in the Park --there was a perpetual military festival. George, taking out his wife to a new jaunt or junket every night, was quite pleased with himself as usual, and swore he was becoming quite a domestic character. And a jaunt or a junket with HIM! Was it not enough to set this little heart beating with joy?
鈥 William Makepeace Thackeray, Vanity Fair, 1848
Junket was once a popular term for any sweet dish. William Adlington's sixteenth-century translation of The Golden Ass of Apuleius (1566) lists a few examples: "Bread pasties, tartes, custardes and other delicate ionckettes dipped in honie." So how did a word for a food become a word for a trip?
The Latin noun juncus referred to a rush (a marsh plant used in the making of mats and baskets). The name junket (likely derived from the Italian giuncata) was used for a kind of cream cheese stored in a container made from this material. From there the word came to refer to the dessert, then a pleasurable outing or feast at which such a dish might be served.