I devoured the Famous Five and Secret Seven series when I was a kid. As with Swallows and Amazons, it was the idea of kids going off on their own and having these great adventures all on their own. With the FF in BOATS. I like boats.
The Famous Five go on a caravan holiday, which ends up in another adventure for the intrepid gang when they meet a travelling circus, where some of the circus folk have more sinister plans than just clowning around.