People plant trees in honor of births, deaths, weddings, and housewarmings. Planting a tree in celebration of a birth is perhaps the most poignant of occasions because this is a gift that will grow alongside the child, year by year maturing and flourishing.
Another great idea is to donate to The National Arbor Day Foundation. They will plant a tree in a national forest devastated by fires and other disasters. No matter how you approach it, choosing to plant a tree to welcome a child to the world is a wonderful, unique idea. This is a gift that will benefit not only the baby, but the world as a whole.