On my last lesson with each class I write a letter for the whole class. You can download examples here and here. The structure is similar for each group but the middle part talks specifically about the class. I mention as many individual students as possible to create a positive story about our journey throughout the year.
After the 'grand reading' I invite each student to write a letter in return. Some students find this hard so there are prompt questions inviting responses but most students ignore these and write whatever they choose. Once the letters are done students are free to choose how we spend the rest of the lesson. (NB: I will try to find some examples and scan them in so you can see the type of thing students write).
Sceptics may worry that at the end of the year (and I usually do this on the very last day) students are too hyped. Not true. Almost every class takes their task super-seriously and writes in silence. For many classes it is the first time I've managed to get them completely quiet all year.
But, if you're still not convinced, below is one of my most favourite pictures ever taken in my classroom. This is a small group I work a lot with and they brought food to celebrate the end of term but just look where their concentration is - even at the end of term!