Humayun`s Tomb is one of the best places in Delhi, If you had not been there by now you should my go there.
It is spread in a wide area and inside it, it is surrounded by greenery, I went there last weekend and advised many people by now to go there.
Its ticket is common 10 rupee for Indian citizen and 250 for foreigner, I totally liked this place and If you will roam freely there it will take around 2 hours to see the beauty, if we talk about its main building its design is somewhat related to Taj mahal.
As you will enter inside you can easily breathe the freshness of that area because of its greenery and in my opinion it is the best place in Delhi.