Anything you want! If you use pipe cleaners or chenille, as they are now called, you can switch out your decor seasonally or by holiday. Hop on over to your local dollar store and see what strikes your fancy.
Here's one I did for Christmas:
Anything. Silk flowers, little birds or butterflies, seashells. I painted wooden knobs to look like mushrooms and added them to one.
Faux lemons are cute on boxwood wreaths: https://www.theaspiringhome.com/diy-simple-lemon-boxwood-wreath/
How about a small nest with eggs and bird maybe a small bird house