Here are the two accessories that’ll give you the biggest video capabilities and quality upgrade.
Tripod: The biggest advantage of a tripod in DIY video is not needing someone to hold the camera.
Microphone: A video with sharp image and terrible sound, sucks. A video with crappy image and great sound, can get buy. Sound is, much of the time, the most important part of video. Meanwhile, the sound on a smartphone or digital camera isn’t too great.
There are two types of mic you can get, the Lavalier mic that clips on a shirt, for recording one person speaking, and then, some sort of mic that you point, of which there are a bunch of types.