What are we doing during independent play?

My kid (16mo) will have some periods of independent play, but needs you to be in the room. I want them to develop the skill, but I can only sit and stare for so long. I don’t want to play with another toy because that will distract them and I don’t want to be on my phone all the time. Aside from reading, what’s a good activity I can do during these times (I’m thinking safe, won’t be messy, easy to stop/start).