It's warm, peaceful and comfortable indoors, so why would you want to take your meditation practice into nature? I practice both indoors and in nature, and I find that each approach offers different gifts.
Mindfulness in nature often uses the same exercises as indoor practice, but involving nature can make a big difference. Some kinds of natural environments can help you be more more mindful: Birdsong, the wind in the trees, or the gentle sound of a stream provide sensory stimuli that are effortlessly fascinating and support quite contemplation.
Nature provides ample opportunities to practise your mindfulness skills. Mindfulness is concerned with focusing our attention on the present moment and nature provides a rich variety of sensory experience Nature can also offer us lessons in acceptance, help us become less judgemental and - by revealing our profound interconnectedness - facilitate greater compassion.
The Buddhist monk Thich Nhat Hanh explains that the purpose of our existence is "to awaken from our illusion of separateness." I've spent many years trying to find ways to help people awaken from that illusion and to realize the glorious truth; we are profoundly connected to nature. Simply being in nature can help deepen awareness of our interconnectedness, but mindfulness exercises are immensely helpful. After years of facilitating nature connection workshops, I've realized that almost everything I teach involves mindfulness.
My PhD research found that meditation was a common pathway to deep nature connection and that it inspired environmental action. Other researchers have found similar results: Mindfulness enhances the impact of our experiences in nature and strengthens nature connectedness. Indoor meditation practice doesn't seem to have the same impact. Although it encourages people to 'think green', that often doesn't translate into pro-environmental behaviour.
If you're already involved in environmental activism, I'd highly recommend trying mindfulness in nature: It can help support you and your work.