Najafgarh Jheel or Najafgarh Drain (its sad its called a drain) or Najafgarh Lake located in south west corner of Delhi is a great place to see flamingoes in large numbers. Apart from flamingoes other water birds are common in this area.
Sandakphu Phalut Trek or Singalila Ridge Trek is one of the best treks that offer breathtaking mountain views. It is the only trek in India where one can see Mt. Everest (Tallest), Kanchenjunga (Third), Lhotse (Fourth) and Makalu (5th) together in a single frame. Spring months are great for trekking as days are longer with beautiful rhododendrons along the trail and warmer nights. However you have to be lucky to get views as clear as winter months.
A short trek through villages on the slopes of Dhauladhar to Kareri Lake with spectacular views. Kareri Trek does not have any demanding days. Excellent connection by road to all over makes this an easy approach to the trail head.