to be very pleasant and friendly, especially when other people are not expecting you to be.
I was expecting her to be in a foul mood but she was all sweetness and light.
be all sweetness and light
To be kind and agreeable. Susan's all sweetness and light, I'm sure she's not mad at you.
be all ˌsweetness and ˈlight
1 (of a person) be pleasant, friendly and polite: She’s all sweetness and light as long as you’re doing what she wants. 2 (of a situation) be enjoyable and easy to deal with: Their quarrel seems to be over. Everything’s all sweetness and light at the moment.