If you love nature, Phong Nha is made for you. Limestone mountains, forests, caves: you'll love it! You don't need a guide, just ask for a map and some advice from where you are staying.
Phong Nha's caves are majestic and if you take a boat trip inside them you can see just how immense the underground caves are.
What I preferred was the eco tourism zone around Nuoc Mooc stream. It was a real adventure because the routes are not well signposted and you have go up some quite steep paths that are sometimes underwater, but what wonderful wild beauty! There were few tourists and the arrangements are harmonious with the nature! At the end of the walk there are two ponds which were a bit difficult to get to when I went there. We swam and there were places where you could jump. Watch out for the river's current! This 100% natural place is right in the middle of the forest and all the green colours are fascinating.
Phong Nha Ke Bang park is full of pretty little ponds where buffaloes drink and children play. It was great fun to mix with them! A motorbike is necessary for getting to these calm and relaxing places.