I try to make most of our work open-ended so we can all stop working at the same time. For example, we write using a workshop model, so they could always keep writing, add details to their illustration, or start a new writing piece.
Depending on what it is, sometimes coloring was the last thing I had them do, so students who finished the task could be coloring.
If it isn't open-ended:
Usually, if we are doing something with paper/pencil, the students who finish continue doing something with paper/pencil such as turning their paper over and drawing or practicing something they need to practice (could be writing numbers, practicing their name, or anything related to what we were working on).
Sending kids en masse to the rug to read books never ended well for me in over 20 years. However, a small collection of books at their table generally has better results.