I've done some very extensive travelling in my short life, I am very lucky.
Peru is gorgeous, you get many lovely ecosystems within a small country. If you have time, you arn't that far from the Galapagos, and that is one of the most wonderous places I've ever been. You also have Maccu Pichu there, fantastic!
Never been to Tanzania, but I've been to South Africa 3 times, amazing place to say the least, filled with life of all sizes and shapes. Africa is always a great choice. Should check out Gabon too, Luongo National park is supposed to be one of the most unique/best game reserves in the world. I plan to get over there soon myself.
Singapore is great, not much nature though, and very developed, but CLEAN!!!! You can hop over to Indonesia/Malaysia/Borneo though pretty easily from there, and the rainforests there are some of the best in the world. There is also wonderful marine life in the area, if you are into that.
Anything else just ask, I love to help a fellow traveller out.