I am studying trees, learning to identify them by habit, trunks, and foliage. It's a bit complicated since all the trees are changing daily at the moment and nothing looks the same from one day to the next. A tree can look completely different naked and with buds, flowers, and foliage. They also vary enormously in their shape, just as no human beings look the same even though we are all of the same species.
I also love the whole history of plant names. There is a large literature devoted to the stories of plants and their place in culture.