1. Cat Ba national park: this island-based park easily makes an impressive add-on to your Halong Bay cruise. The park is a tropical primary rainforest with a large collection of animals, birds and reptiles. Many of the plants are unique to Vietnam and very well protected. There are a number of trails, such as those that lead to Ngu Lam Mount, to Trung Trung Cave, and Frod Lake, each offering a distinctive experience. There are also swimming opportunity though check the weather in advance. The Park is good for a day-trip but you can sure bring your camping equipment if you have more time to spare.
2. Cuc Phuong national park: an off-the-beaten path option, Cuc Phuong park is located in Ninh Binh province, close to the Hoa Lu ancient capital. There are many tours on offer from Hanoi that involve biking along the scenic view of lime stone mountains and rice paddy field. Otherwise you can book a minivan directly to the park. It is considered one of the best places for bird watching in Vietnam. Its almost out of the way location means that you are less likely to run into tourists and enjoy an authentic natural experience.
3. Phong Nha- Ke Bang national park: Phong Nha- Ke Bang deserves more attention that it does right now, for its impressive lime stone cave complex. Taking a boat ride along the river, you will be pleasantly surprised by the tranquility of a countryside area mixed in with the nature wonder of caves formation. Avoid the summer time, when domestic tourists are crowding the place.
4. Cham Island: An hour by boat away from Hoi An, Cham Island is recognized by UNESCO as World biosphere reserve. It is the greenest place in Vietnam, where the use of plastic bags is completely abolished. People who live on the island are extremely friendly and you will sure enjoy yourself heartily with the sun, sand and sea.
5. Cat Tien national park: If you are in Ho Chi Minh City, you might consider spending two days at Cat Tien park. About 150km to the North of this largest city in Vietnam, Cat Tien is one of the best preserved parks in Vietnam, being a symbol of bio-diversity. Cat Tien is home to 40 species listed in World Red list of threatened species. There are plenty to do, from paddling along the river, hiking, cycling to camping. Ask for a map when you buy the entrance ticket.