I don't think Cochin has any official trekking spots. Me myself being a native of Cochin I have not come across any Trekking spots in Cochin except for some roads which provides a real trek feeling!! ;)

Well I am also interested in treks, and have been to a beautiful trek location in Idukki. I don't remember the name of the hill now, it was a Christian pilgrimage spot. I think the name was 'Kalveri'. It is just above the periyar river and the trek is real hardcore. There are even life threatening trek paths. It will take an approximately 8 hours for the trek. It is a thick forest and will be very exciting. The scenery from the top is breathtaking.

Munnar Hills - one of the most popular hill stations in India is situated at the confluence of three mountain streams - Mudrapuzha, Nallathanni and Kundala. Located at 1600 m above sea level, this was once the summer resort of the erstwhile British Government in South India. Sprawling tea plantations, picture book towns, winding lanes, trekking and holiday facilities make Munnar a unique experience. Munnar also has the highest peak in South India - Anamudi , which towers over 2695 m. Anamudi is an ideal spot for trekking.

To know more you can visit this url
http://scstsenvis.nic.in/envis/new_page_59.htm