Generally, I'm not a big fan of writing exercises. I would rather be writing something I plan on doing something with than performing the writerly equivalent of navel gazing. Probably the worst of these is the writer's prompt. Search for writer or writing on Pinterest and you will be spoon fed numerous prompts that are supposed to inspire or give practice. But really. I don't want to practice writing another person's idea.
But, there are things that writers can do that will lead to bigger and better things. I call it sketching because it is essentially what an artist does. Head out with pen/pencil and paper and find things to sketch.
One thing that I like to do is head out to a cafe or park or some other place where people meet up with friends and find a couple of group of friends and sketch out the conversation they are having. I don't eavesdrop, though sometimes I will hear a bit here or there. What I do is watch them for a minute or two. I look at how they use their hands, hold their heads, their lips move, and the way they carry their bodies and dress. Then I sketch out their conversation or the hidden thoughts that each is thinking to create a little scene. It doesn't have to be much, certainly not pages, just a little bit on how I interpret the scene.
This makes great practice for characterization. All the details of the colors of their clothes, gestures, and especially what they do when listening help improve scenes that I intend to write. It creates a catalogue of reference scenes, I suppose.