I guess you'll be flying into HCMC, so that holds a number of interesting things to check out.
Just north of HCMC is a place called Cu Chi Tunnels. They are a system of tunnels/bunkers that the Vietnamese used during the war.
Just south you have the extensive Mekong Delta, offering lots of nice little places to visit and is very beautiful.
To the east you have some beach resorts. Mui Ne is also a very picturesque place. There is a small desert there that offers activites like dune buggying, ATVs and sand sledding. Also beachy stuff like kite surfing.