Travelling with 8yo son, Southern Thailand/Malaysia Options

Hello, I am planning a 7-8 week trip later this year with my wife and 8yo son. We have travelled extensively in SE Asia but have never visited Southern Thailand or Malaysia. We'll be coming down from Yunan, into Laos, and then following the Mekong to Cambodia. I'm looking for some recommendations for the following criteria:

- Southern Thai islands and mainland that offer nature, kayaking, waterfalls, caves, temples, fun activities for kids. We are no interested in busy resorts, nightlife, lounging on a beach.

- We'd be travelling into Malaysia from Thailand, any recommendations for similar experiences as we travel down to Singapore (east or west coast?). We'll likely visit Penang and Langkawi but otherwise are undecided.

- Is it worth skipping the peninsula and heading to Sabah/Sarawak instead? The longer travel times are a slight concern as well as the cost of guided trips into the National Parks.

Any help and suggestions will be most welcome. Thanks.