For most, the perfect end to any holiday is a few days on the beach and from Saigon you have 3 very different choices. Mui Ne / Phan Thiet are on the same bay 5 hours by train or car from Saigon, perfect for those who'd prefer not to add further flights and would like a more established resort with plenty of restaurants and bars. Alternatively an hour's flight takes you to Phu Quoc Island or the Con Dao Islands, still relatively undeveloped beach destinations ringed by beautiful beaches. The 4th option is to fly an hour north to Nha Trang, boasting beautiful beaches and luxury hideaway resorts. Alternatively miss out Saigon and stay in Hoi An, upgrading to one of first class hotel's on the beach.
Day 1: After 3 nts in Saigon fly or drive to your choice of beach resort for 3 nights or more
Days 2-3: 2 days to relax on the beach, enjoy the hotel's facilities & perhaps take an optional excursion
Day 4: Perhaps extend beach stay? Otherwise afternoon airport transfer and overnight flight home