Some of us have a little birdhouse in our soul and others hang them in the nursery. They're inexpensive, completely customizable and what could better symbolize the "nesting stage" than a birdhouse?
If you really wanted to start from scratch you could build your own birdhouses, but they are also readily available (and inexpensive) at craft stores like Michael's, Hobby Lobby, etc. Decorate with paint, washi tape, or decoupaged paper or fabric. Some parents even made them into small night lights.
Jane's Coral & Maize Charmer on Apartment Therapy
No Gray Skies for Mallory Gray on Apartment Therapy
My Room: Charlotte Grace on Apartment Therapy
Victoria's Room by Sissy + Marley
Emerson Kate's Soft, Vintage-Feel Nursery on Apartment Therapy
Frances & Jane Create from Scratch for a Growing Family on Apartment Therapy
Ava's Bohemian Bedroom on Apartment Therapy
Livy's Nursery on The Caldwell Couple
Layla's Small, But Stylish Aviary on Apartment Therapy
My Room: Amos on Apartment Therapy