I remember reading a story about some kids that have an adventure somewhere and part of it involved riding in a canoe (or similar), inside a mountain, on a river that flowed through it in a sort of cave. Sometimes they would stop and hideout in the passage way, but it led to a secret place (I want to say jungle).
Oh god, its so vague.
I think there was a couple of books, not necessarily the same chatacters or places but similar style all same author.
Very similar to Endi Blyton type books.