For natural-world immersion – crocodiles in the pond, monitor lizards in the grass and occasional visiting elephants – the Jetwing is a great choice. The huge, individual chalets (book a 'water' or 'forest' dwelling for the best views) have a rustic look outside, but inside they are equipped with all mod cons (including Bose sound systems).
A resident naturalist is on hand to give walking tours, there's a yoga instructor, and the cuisine (and dining room setting) are superb. It's 7km west of Sigiriya.