You can still do things even if your character isn't. A big draw of D&D is the social aspect, you can still roleplay reacting to being hit or seeing an ally do something.
If you want to exercise your brain while waiting on your turn do like high-level chess players and think during other people's turns, I find that fun. Think how the recent decisions or rolls modify the state of the battlefield or your chances of victory.
If you want to exercise your brain while waiting on your turn do like high-level chess players and think during other people's turns, I find that fun. Think how the recent decisions or rolls modify the state of the battlefield or your chances of victory.