During my trip to Vietnam, I stopped at Nha Trang during a bus journey between Dalat and Hoi An. It's not somewhere you necessarily choose to go when you go to Vietnam but Nha Trang is a beautiful seaside resort whose white sand beaches have views over the turquoise water of the China Sea. And what's more, there is bright sun 9 months a year, and even in the rainy season it doesn't rain all the time.
Apart from the beach, I recommend you visit the market. Its quite typical but very touristy. The fishing port with all its blue boats is also worth a look. To finish up your visit to Nha Trang you really must go and see the Po Nagar towers. They are the most beautiful remains of the Cham civilisation. Not only is the site very beautiful but the view over its surroundings is marvellous.
I have a somewhat mixed opinion on Nha Trang. At first glance it seems to be a coastal resort town that you might find in the south of France, with luxury hotels, a water park, nightclubs and even bars where hookers ply their trade. In the downtown area, all the signs are written in Vietnamese and Russian! The vast majority of the tourists are Russians: nouveau riche and frankly, not very likable! Everything stinks of money and the locals greeted us in Russian... it was horrible.
Despite that, when you escape from the downtown area, you discover the real Vietnam. The market is spectacular, the people are much more natural and you return to Asia after leaving Russia behind.
An unexpected bonus: the massages given by blind people, which are very powerful and effective.
On the downside: Everything is built for the Russians and there is an airport with direct flights to Russia!