Check out the Tiger Temple in Kanchanaburi which a couple hours coach west of Bangkok (the temples real name is Wat Pa Luangta Bua Yannasampanno but don't go trying to pronounce that...). Basically the temple contains abandoned / orphaned tigers which have been brought up by the monks. The tigers are adjusted to human company and you get to walk among them / touch / get photos with them in the nearby canyon. There is a "donation" you have to pay when visiting the tigers (about $8 USD) but keep in mind that all this goes to the care of these tigers and helps support the continued rescuing of more.
While in the area, check out Erowin National Park - take the walk up past the 7 waterfalls and make sure to have a swim in whichever pools look the nicest (as long as you don't mind having your feet nibbled on by the fish on the way out), and stand under a few of the waterfalls - it's invigorating :). about 10 years ago
Eco-trekking in Chiang Mai. There are tons of outfits that take you trekking, but steer away from all who offer elephant rides, floating on rafts, 'ethnic shows' and so on - hardly authentic and very touristified. What you want is a small outfit that takes you and your (small) group to a remote village where you are the only Westerners, and where the villagers dont put on a show for you. This should also include some trekking, the length of which depends on your schedule. It is worth asking around and doing some research in Chiang Mai to find a guide and tour you're comfortable with.
(trekking can be arranged in other cities in Northern Thailand as well) about 10 years ago
Get away from the tourist crowds and head to the rural northeast, and stay in a rural village. Here eco is not a byword but a necessity: harvesting rainwater in areas where there is no mains supply, looking after the land so produce is sustainable, etc. Some places such as Gecko Villa (www.geckovilla.com)aim to help the local commuity by creating employment and encouraging reforestation, recycling etc.
Andaman Discoveries is another option in the south of the country: one of the longest established eco outfits. over 9 years ago