It doesn’t have to be big. Just one thing from today is enough. The bus showed up right on time, someone held the door for you — that’s plenty. You don’t have to go searching for it. If there isn’t one on a given day, just skip it.
But if you jot one down for a few days, you’ll start noticing one even on the days you thought had none. And you don’t have to comment on other people’s posts, either. Just reading them is enough.