A day is enough time physically to see the sights in the city proper. You could do a day trip to the tunnels, another to the Mekong, and do food and walking in the evenings. Or some Mekong tours are several days if you wish. But either way, I'd not budget more than 3-5 days max in SGN, and that's only since you like walking around a lot. For most people I'd suggest 2-3.

You could consider Cambodia and Angkor Wat, a few beach locations, or some of the cultural places in Central or Northern parts of the country.

ETA: If you come through without a visa, i.e. Europe or Oz passport or something, then go out to Cambodia or elsewhere, you might not be able to come back into Vietnam. There is a delay before you can use the waiver a second time.