Besides art there's an endless list of what you could post. Random thoughts, cool pictures you've taken in the past, talking about life & opinions, really anything that comes to mind and that's what most people do anyways. And don't worry, there are a ton of artists out there that also take weeks to upload new stuff, meanwhile they just do some of the things I've mentioned, retweet other peoples art, go into comments on a tweet and so on. That's all there is to social media in general, there's no golden guide on how or what to post, just be yourself, follow some people you find cool and the rest will come naturally.
