What is that thing around Hanfu's neck?

The thing around Hanfu's neck is a wreath.

Wreath, also known as Artemisia annua. Han costumes.

It is an ornament hanging around the neck and on the chest, which consists of various small flowers. It is an ancient Indian style, which was introduced to China with Buddhism in the Han Dynasty. In painting, it was used by Buddha statues, immortals and goddesses, and later became an ornament for ladies.