Had a great stay at Niramaya with Friends in big family villa! Hotel is probably the best villa type accomodation on Koh Yao Noi - luxurious, quiet, clean, modern design though with beatuful Thai decore. Staff is very friendly, attentive and helpful. The only minor issue we had in our villa is mini floods in bathroom, caused by water drain problem - hope it will be fixed fast.
Restaurant is also very good and has reasonable prices. Few options are walking distance - especially we liked the Pyramid restaurant who managed to organize amazing seafood dinner on a beach.
Beautiful beach is nearby, but corals make it not very comfortable to swim. Anyway, villas have private jacuzzi and big pool.
Island itself is very small and can be explored by bike within half a day. Bike is a must unless you want to stay in hotel only. Very relaxing atmosphere everywhere.