Alibag...merely 100 odd kilometres from Mumbai by road is known for a nice plain beach. Alibag has easy access by MSRTC buses, private/tourist taxis and also launch service from Gateway of India.
This place also has historical monuments around, which stand unfortunately in poor state due to negligence of local authorities.
There is fort Colaba about 1km inside the sea. During low tide, you can walk upto the fort, spend some time and return before the high tide starts. Lot of tourists have been ignoring the warning notices about low/high tide timings and have faced death in recent years.
In the city of Alibag, once can see the monument of Sir Kanhoji Angre, who was the chief of aarmaar (navy) of Shivaji Maharaj.
about 12km - Revdanda port
about 40km - Murud Janjira fort
about 14km - Birla temple built recently by Birla group
about 8km - Kihim beach with MTDC resorts / tents
If you have been thinking of going there, make it before monsoon. However, best time is between Oct-Jan.