We found a great place to trek in Delhi - Asola Wildlife Sanctuary on the Aravili Hills.
Options: There is a well defined trail for few kms or you can go a little adventurous and find one yourself. But do carry a map enabled mobile phone or GPS device as this place is quite large and isolated.
Start: You can start from the Asola Wildlife Sanctuary gate which is few hundred metres away from the Karni Singh Shooting range on the Suraj Kund Road between Tuglakabad and Suraj Kund.
What to do: This is not a well defined trail, so it promises a lot of adventure. As soon as you enter through the gate, you can park your car on the left-hand side and take the trail from the farther end of the parking lot.
Follow the trail next to the fence and it will take you to the lakes (behing the Kant enclave).
If you are a bit more adventurous, you can deviate from the trail and find your own way. But ensure you travel in group and carry a GPS/map-enabled-mobile device for directions.
Best time would be to visit in winters.