I found a wonderful 21 day happiness course (https://www.tes.com/teaching-resource/ks2-21-day-happiness-course-for-kids-and-teachers-really-works-11205744), which I personally use daily. However it is not very useful for younger children.
I rewrote it, getting the students to think about 4 things which are good in their lives and one moment of happiness from the current day. You would be amazed how hard this is for children to do. By doing this each day, every day for 21 days, picking out the positive and happy moments becomes a part of their every day existence.
It has an additional benefit in the fact that this is also an excellent point of time for teachers and children to discuss each day what makes them feel good and what went well on the day. I would recommend this as an afternoon activity.
To see examples of my own journals and those of my 6 year old son, please visit my blog at http://www.Barefootbuddies. com. Please leave a comment below to say how it went in your class.