Here are some awesome picnic spots to visit in mumbai
Lonavala 84km away
Elephanta caves 22kms away
Alibaug 96km away
Palghar is 87kms away
Manori 63km away
You can cover the following places as a one day picnic.
Sanjay Gandhi National Park
Elephanta Caves
Karjat
Karnala
Lonavala
Manori Island
Pawana Lake
Mandwa
Igatpuri
Jawhar
Revdanda Beach
Lavasa
Panchgani
Kolad
Mahabaleshwar
Alibaug
Rajmachi
Matheran Also Maharashtra is popular for trekking activities. There are some of the trek which you can cover in one day.