As mentioned in my e-book, "Hygge in the Classroom", Children benefit hugely from taking ownership and responsibility for their own actions and behaviour and the more positive the approach, the better for everyone.
This poster should be displayed each Monday morning, enabling your students to think about how they can be marvellous that day. Perhaps they could note down an idea in their jotter? Throughout the day, if you, support staff, lunchtime supervisors or, indeed, the pupils themselves spot anyone being 'marvellous', then they can note their name on a post-it and stick it on the poster!
At the end of the day, the children should be congratulated and a photo taken of the poster, so that their achievements are not forgotten.