Here are a few useful phrases to help you talk about "la rentrée":
Did you have a good summer?
Did you go away?
How was the weather?
When did you get back?
I had a (really) great / good / terrible time in ______. I.....
I feel / don't feel _________ to be back at school / work. I....
Don't forget, the formula for successful small talk is this:
Ask a question -> Answer. Add some more information. Ask another question.
Ask -> Answer -> Add. Repeat.
It's simple and you'll keep the conversation going.
As they say in France "Bonne rentrée everyone!"