Don't look any further than Evergreen Ecolodge in Chitwan.
If you are visiting Chitwan National Park in southern Nepal, you will be staying in the town of Sauraha. As Sauraha is less than 2 hours away from our home in Hetauda, we have visited a few times and stayed in different places.
The best place we have stayed in or seen in Sauraha is hands down Evergreen Ecolodge for the experience and for the unique concept.
We have always loved off-the-grid living as outlined in our 40 LifeHacks eBook: http://thetickettotravel.com/product/40-lifehacks-that-save-over-2000-dollars-per-month-ebook/. Earthships and eco-living has always been cool to us. With local building supplies, recycling measures, solar power and sawdust toilets, these are the kind of environmentally concepts that have been implemented in Evergreen Ecolodge.
And with the nest experience (the little hut in the middle of the thumbnail picture) for only around $4, it is a very inexpensive experience as well.
It's in the middle of town, not far from where you'll want to go. They can help you to organize activities that will respect the animals inside the park, especially the elephants which do not always have kind-hearted trainers.
For more information you can check out their website: http://evergreenecolodge.com/.