Malshej Ghat for an early morning drive.
Lonavala for a one night stay.(Della Resorts)
Then there are lots of treks particularly for the monsoon season, check out This guys on his own trip page on FB or some other google searches for water rappelling and treks.
Lavasa, Karjat, Bhandardara for treks and all.
You can try Manori. a small village which lies in the northern part of Mumbai. The village is famous for its beach as well as Manori creek. Away from the hustle bustle of the city, the place is entirely peaceful and perfect to vent out your stress and go into the relaxation mode.