If you’re environmentally-minded, there are only a few options for certified “green” hotels in Paris, but they’re good ones. Right at Trocadéro (just across the Seine from the Eiffel Tower) is the Hotel Eiffel Trocadéro (€200-€300/night) and its sister hotel Le Gavarni (€160-€260). If you like the idea of escaping to a private hotel garden after a day in the busy city, and don’t mind being in a quiet-but-chic residential area, try the Regent’s Garden Hotel (€170-€250). For those on a really tight budget, the Solar Hotel (€89 flat rate all inclusive) is a great eco-friendly option right off a bustling market street near the Catacombes and Montparnasse Cemetery (they also have free bike rentals). Please don't forget what you save in price you make up for in space!
For those who like the wide selection offered by sites like Booking.com, try BookDifferent.com, a site that uses the Booking.com database but donates 50% of its commission to one of the 126 participating charities (you get to choose). It let’s you sort hotels by carbon footprint or whether it’s eco-certified, and tells you exactly how much of your stay will be donated to the charity. Of course, it may make more sense to just book direct with the hotels (the hotels give preferential treatment for those who do), and donate the saved money direct to one of your favorite charities. But if you want the “path of least resistance” option then go ahead and reserve with BookDifferent.com.