I've come to the decision that there is nothing of intellectual substance that can be done in under two minutes, which is the amount of time I am most often looking to fill on my smartphone.
At best, I can review things and eliminate the ones that don't need attention: emails I don't need to read, RSS reader posts I'm not interested in, notifications that don't require a response.
But even those tasks only take a few seconds when the time comes to read email or RSS. So I haven't really accomplished much.
Anything else, whether an email that needs a response or an article that deserves attention and consideration, takes time. If you do try to wedge them in to 3 spare minutes between other tasks, you feel like you Got Something Done but the thing will be done poorly or incompletely. Either you send off a poorly-written email, or you have it half-done and have to remember to finish it later. It's a no-win.
Social media used to be the thing that filled this gap. Any time you have time, pulling the slot machine handle to see if anything engaging to the limbic system had happened in the last 45 minutes since I last checked.
Since I took those apps off my phone, I've had to retrain my reflex to pull the phone out any time I have 30 seconds otherwise unoccupied. Remind myself this is too small of a slice of time to meaningfully process any input.
Instead, I try to reinforce things in short term memory that I think are worth remembering: useful lessons from a book, or a question I've been pondering the answer to. Often in this mental quiet period, I'll remember something I'd made a mental note to do, and be able go do it, or actually make a physical reminder to follow up. If I was reflexively panning for limbic gold on social media, I would be too distracted for any of that to happen.