It's always best if the first note home is a positive one. I"m sure you know of many positive reasons, but here's a quick list to keep in mind.
Jumped up a reading level.
Finished a book during Writers' Workshop. Ask parents to look at the book and read it with the child at home.
Earned high marks on a test.
Learned a new skill.
Improved in a subject area.
Good character (witnessed the student being caring, honest or thoughtful, etc.).
Tell parents to prepare for student of the week (send a photo and fill out a form to share).
Remind or thank a parent about his/her scheduled volunteer time.