Regular nair, DJC nair, or FC nair OOS are all good options. But, FC Nair is only 1 frame slower than DJC nair if you do it right, so there really is not reason to use DJC nair rather than FC Nair if you can actually do FC Nair. Also FC Bair is good if your opponent is pressuring your sheild from behind. A good mixup might be FC Uair. Uair actually covers a lot of distance above and to the sides of you. It's very underrated IMO.
Also, you could dsmash oos. that might work well as a mixup. If you don't know how: Doublejump land, let go of trigger, then press down on the c-stick. It's kinda tricky timing and you have to do everything pretty quick, but if you do it right it comes out in like 10 frames or close to it.
Also, Upsmash OOS might be good, but situational. It does have small hitboxes on the sides which could lead into a grab or dash attack or FC nair