Nha Trang is a beach area. Other than enjoying the beach, there isn't much there. But every town has markets. In some places, the local markets (where the locals shop for food) are only open in the morning. Given that Nha Trang is a tourist town, I expect it has some kind of market open all day.

there's a huge aquarium

We paid a visit to the morning market down by the river which was mostly fish as I remember en route to some Cham ruins close to the town. I am sure you could haggle with a taxi driver to take you to both. Not sure whether there will be much to see in the pm.

There is a famous Vietnamese photographer who is NT born and has a gallery in town which is worth a visit. We bought a number of B&W prints from him - one of his photos used to be featured in the LP guide. Close to there is embroidery / needlework museum which is worth a quick look.

You could also rent a boat to go upriver for a few hours through the countryside which is quite pretty to the railway bridge, although I should perhaps point out that ours broke down mid stream and we were stuck drifting along for an hour or so until the boatman got a signal on his cellphone to call for another boat!

Nha Trang is considered the most famous seaside resort-town in Vietnam so there are lots of things to do. However if you stop by half a day, just enough time to enjoy the beaches, swimming and sunbathing. Nha Trang has many markets right in the center of the city, open all day. Dam market (Cho Dam) is the biggest one where you can buy fresh fruit, vegetables, fish and squid from the sea, other raw products and souvenir.