The remote for certain, a bag to carry the drone and any extras. You can fly the spark with a phone but with really short range only, you need the remote. So those things are minimum. But you’re better served with an extra
battery or 2. Probably a gimble cover, which is cheap.
After those things in my opinion, extras are just based on personal wants.
I too bought the spark as a small and simple flying camera, and it works really well for that. Aside from the spark I had what I listed as musts, and a few other smaller items. That was enough.
Should mention mine disconnected and flew away, which happens. Learn all about the drone, and consider getting one of those tile type of small tracking devices to attach to it somehow.
Although not a must, for me personally, my next drone will fly with a tracking device.