You can’t miss the Cu Chi Tunnels if you are in Ho Chi Minh City, previously known as Saigon before 1975. It is located about an hour and a half from the city centre by bus. The tunnels are part of an amazing underground network constructed by the Vietnamese to fight the American soldiers during the Vietnam War. Today, it is a very important part of the Vietnamese history for the younger generation of locals and foreigners learning about the Vietnam War.
I would recommend to take the half-day tour from one of the many travel agencies located in Ho Chi Minh City. It costs around USD 4-5, depending on you haggling skills. The price includes the journey of the bus to and fro and a tour guide included. It is also faster and less of a hassle. If you decide to take the public bus, it is slightly cheaper which comes to around USD 1-2. Entrance fee is excluded from the prices mentioned above, which is about USD 4 at the time of writing.