Stretch with the class to start the day. Have everyone stand up by their desks and reach for the sky. Then instruct them to slowly bend down and touch their toes with their fingertips. Flexibility is an important attribute for kids to have health-wise, and the stretching also helps them wake up if class is early in the morning.
Take the students for a short walk every few hours to get them away from their desks and moving around in a constructive way. Walk laps around the school if you're able to, or have the class walk in circles around the room if you aren't able to leave.
Bring students to the front of the class to write on the board or answer questions. Even the short walk from the desk to the front is exercise, because the body burns calories when it is in motion, and the movement helps prevent the child's mind from wandering during instruction.
Play a game of charades with the students based on the subject you're studying. Give different students a chance to lead a round. Have everyone stand while the game is being played.
Create a fitness alarm, which is a bell that you ring at random times during the day that indicates a physical activity is required. For example, every Monday, ring the bell when you want the students to get up and do 10 jumping jacks. Get creative, and ring the bell at least twice each day to keep the kids moving.