I particularly enjoyed visiting the Museum of Cham Sculpture: it's educational, and some of the information on display is in French. There's a view of the Dragon River Bridge to enjoy once you exit the museum.
Another thing that sticks in my mind is the entirely pink-clad cathedral, which is very pleasing to look at! I didn't have the opportunity to go inside however (be sure to check the opening hours beforehand).
The best time to visit the Marble Mountains (located 12 km from the city centre) is either very early in the morning or close to closing time, as it gets very hot there. What I particularly enjoyed at the mountains were the views and one of the grottoes, which was quite spectacular. I never imagined there could be so many temples and pagodas at this Buddhist sanctuary, so high in the mountains as it is! It's something of a workout getting up there, but it's definitely worth the effort!
The Lady Buddha Statue (Linh Ung Pagoda, located 10 km from Đà Nẵng city centre, is a religious site from where there are wonderful panoramic views over the city and the sea. It's actually located on a peninsula, and when I was there I got to see the views at sunset, which was simply magical! Đà Nẵng's pretty beach is located just next to the site, though in my opinion it is not one of the most beautiful beaches the Vietnamese coast has to offer!