In the past cooling down involved a bit of post-workout stretching. Things have changed.
Standard cool downs usually involve:
Pulse lowerer (approx. 10 minute) of tapered light intensity CV activity to avoid blood pooling and to remove the lactic acid backlog built up in the muscles.
Maintenance stretching i.e. deep static or PNF work.
More advanced approaches usually involve:
Self myofascial release (on a foam roller or small ball) or light sports massage