Secret passages and tunnels/caves, it really is becoming the trademark of Enid Blytons Famous Five

They do start outdoors though, camping in a caravan, not much more outdoors you can get.
The circus people they meet are quite old-fashioned but realistic, the same goes for the treatment of the children and animals.
A couple of dogs next to Timmy of course, a chimpansee, an elephant, many horses, ... plenty of animals populate the pages of this book.
The four childres with big Timmy as a guardian are allowed to take a holiday without accompanying adults, roaming free in the nature in two caravans pulled by some older horses. A dream for any child which explains its appeal to the audience of course.
Friendly farmers assure a steady supply of food, food that is consumed in large quantities by the five.
Youth sentiment and feelgood sunnarize this great story.