I think you could spend 2 days in Da Lat. Rent a bike and drive around. Da Lat is best on bikes.

But before that check out Hue first, give it 2 days. Eat a lot! Hue has very good (the best) bun bo (beef noodle) and banh beo (small rice cake), although sometimes their dishes are pretty salty and spicy to my taste.

If weather get better, check Hoi An out. A lot of my friends (locals and foreigners) enjoy the flooding scene in Hoi An. Leave your luggage safely in Da Nang. Remember to check ahead with your accommodation if they are flooded.

The national highway was damaged in parts but service is already recovered. Airlines works fine now though delays should be expected.

After Da Lat, travel to HCMC, give it 2 days. Try Me Kong delta too.