A Haiku is a short, 3 line poem made up of a basic 5 syllable, 7 syllable, 5 syllable pattern. You could brainstorm some things to be thankful for or feelings of happiness and joy you'd like to celebrate.
Then try using Haiku together to express these feelings! For younger children this could be led by a teacher or parent with kids just giving words and feelings as suggestions eg "happy", "laughter", "friends", "cuddles" etc.
Older children could have a go at making their own after having it modelled to them.
If Haiku proves too complicated then perhaps just make a collection of happiness words to display at home or school instead?