How about some music games? Music history games; theory games; anything you don't have as much time for when lessons are in full swing.
For example [...] write a short composition based on an summer vacation or activity; Improvisation based on a chord progression from Pattern Play or other source.
Composing and improvising are great ideas! And, speaking of Happy Birthday, consider having your students compose a harmony to the melody. They can make it as simple or complex as their abilities allow. Over the weeks, they can work on memorizing and maintaining it so that they're ready to play Happy Birthday any time there's occasion to do so with a piano nearby.
It might be fun to update the harmony once a year or so, or add new flourishes to the melody, or what have you, that will serve as a nice record of growth in musicianship over the years. (Showing increasing understanding of rhythmic and harmonic textures and complexities as time goes on.)
That last part was off the top of my head, as I've only had my students write their own harmony to Happy Birthday one time. (Which reminds me I haven't done that yet with a couple of students.)
Have fun, pianist lady! Enjoy the fresh start with your returning students.