While trekking on Marangu day hike to Mandara hut, you will cross the rain forest which is a dense mountain forest with pure green vegetation and a lot of species of plants.
If you are lucky you can see Blue Monkeys and a black and white Colobus Monkeys. The extra short walk to Maundi Crater offers a good view of Kibo and Mawenzi peaks and also a good view of the Kenyan side lake Chala and Jipe (all on clear days).