2. Your former boss
It 's a good way to keep relationship with your former boss by writing greeting card. Who knows if you will need him to write a letter of recommendation, introduce you to work or hire you back to the company again!
Happy holiday! I hope you all go well and wish success in your work. Thank you again for having a very pleasant time at (name of your former company).If you are busy with the end of the year, I hope we can have time to drink coffee / make a phone call / eat a meal, talk about each other's current situation. I am happy to know what you are doing recently.