We are flying to Da Nang tomorrow (Sunday) for 5 days.  I'll need to exchange a little bit at the airport, but once I get to the city where will I receive the best exchange rate?

I don't know about the "best but it is very easy to change at any jewelry store or places in the hood. Just ask at your hotel. they are also not very pickey about marked up bill ect.  If you are staying by the beach you can exchange with the motorcycle rental guy in front of Samatha Apartment and hotel. His rate was better than daily rate on internet and his rental bikes are in very good condition.I would pass on staying at the hotel.. It's very easy to change money but you can compare rates before  a large exchange. Forget the banks

I've seen the gold stores in VN doing exchange.

I spent 4 months in Na Trang in 2019. I always used gold shops and always achieved an excellent rate.