If you are looking for something to do in around Mumbai in Monsoon then visiting Manori beach could be one of the options.
Although there is no over selling point here as the beach is pretty secluded with almost nothing around but can make up for a good time by not leaving the city limits.
If you enjoy drives and sea then this could be a day trip for you. The beach is 5kms away from Gorai beach, where most of the people go. Gorai tends to get crowded on holidays and weekends but Manori manages attract less people which made it perfect for me.
Things to remember:
The roads are narrow and there is hardly space for 2-3 car parkings. Be ready to park the car a little away and walk to the beach, many come here on bikes as well.
The beach is not very big but you can sit on the rocks or on the church which is at the entrance and enjoy the view.
I am being brutally honest and telling you that the water is not exactly clean, so you need to pick your spot and park yourself there.
The beach almost disappears during high tide.
You can pack a picnic box and then lay out a picnic mat on the beach; it would make up for a really nice memory by the beach.
The road to and fro the place is beautiful and you can definitely wait and click some good pictures.