Nha Trang is a different city to many in Vietnam, it has already become commercialised for the high volumes of tourists that come in. It reminds me much more of a European city than Vietnamese, at least in the coastal area I was staying. The prices are higher and there are a lot of chain shops and restaurants.
The main thing I did on my full day in Nha Trang was ride out to Ba Ho Waterfalls, which is out of the city but worth the trip. It's a series of waterfalls that you can swim through and jump off, with an inflatable obstacle course further down the river near the restaurants. The drive out there is also amazingly beautiful, along the coastal road.
Aside from more Banh Mis and the standard fruit, bread and egg breakfast from the hostel, the dinner was in a German owned restaurant called Haus Bremen that served enormous platters of meat: perfect for recovering from a day swimming in the falls.