As you progress through the Challenge and clock your progress on the Eat, Drink, Shop Healthy Challenge Passport, there are selected positions on the Passport which would grant you additional rewards on top of the baseline Healthpoints for each stamp.
These stamp positions are marked with the reward icon

on the Passport and the rewards could be prizes, e-vouchers and/or bonus Healthpoints. The rewards vary across different stamp positions.
To identify the rewards that you can stand to win, tap on the stamps with the reward icon to view the list. If there are more than one reward listed, you will win one of the rewards on a randomised basis, upon collecting sufficient stamps to land on the reward icon.