Murud Janjira is perhaps an one of a kind of fort. Now with a road coming up from Pune to Kolad on the NH17, its easier for people from there also to visit this unique bastion of sea prowess of the Siddhis and the Marathas.
While its just a day journey from Bombay, if you stop to take in the other places on the way, its going to be a week-end getaway. The places I would recommend stopping are all south of Alibag on the road to Murud Janjira.
Immediately after you cross the narrow streets of Revdanda and get across the bridge over the river, ask for Korlai beach. The fort can be seen from the road, but you will need directions. The beach is deserted and white. The fort atop the hill has a lighthouse at the bottom and can be approached by car - though carefully.
After Korlai, stop at Kasid. The drive from Korlai to Kasid is also scenic with the sea barely few feet from the road at places. The approach to Kasid is to be experienced and cannot be described. The beach is white and set in a crescent shape of the coast. There are also two reasonably good hotels thankfully away from the beach for refreshments. In fact I would recommend staying at Kasid and making trips to Murud Janjira instead of staying at Murud Janjira town.