A Lazy Afternoon is a short story by Enid Blyton following the further adventures of the The Famous Five. It was originally published in 1954 in Enid Blyton's Magazine Annual #1; as this was the same year as Five Go to Mystery Moor (and assuming that this is an indicator of internal chronology) this story might be considered #13.5 in the series.
This is a fairly decent story, although at 20 pages it is quite limited in scope. It is vaguely reminiscent of Blyton's 1956 book Five on a Secret Trail, although it would be a stretch to claim a definite link, and the short story is lacking in most of the elements that made the book engaging. Overall, this is a fun reminder of the Famous Five, but doesn't hold a candle to a full novel.
Minor spoilers: While on summer holidays at Kirrin Cottage, the Famous Five take a walk into the local woods to escape the heat. While napping in the shade they see some shady-looking men drive up on a motorcycle and hide a suspicious-looking bag. They apprehend the thieves and are heroes!
A Lazy Afternoon is present in most (perhaps all?) Famous Five short story collections, including Good Old Timmy and Other Stories, Five Have a Puzzling Time and Other Stories, and The Famous Five Short Story Collection. It is also available as a stand alone "book", one of the Famous Five Colour Short Stories.