There's a special temple stage hidden within each section of map. In order to find them, you have to collect the K-O-N-G letters from every level of that particular map section.

Temple	How to unlock
Beach Temple	Collect all KONG letters from every Beach level.
Cave Temple	Collect all KONG letters from every Cave level.
Cliff Temple	Collect all KONG letters from every Cliff level.
Factory Temple	Collect all KONG letters from every Factory level.
Forest Temple	Collect all KONG letters from every Forest level.
Jungle Temple	Collect all KONG letters from every Jungle level.
Ruins Temple	Collect all KONG letters from every Ruins level.
Volcano Temple	Collect all KONG letters from every Volcano level.