Daman is a really good get away destination. Especially from Mumbai. The nearest rail point is Vapi in Gujarat where most of the trains stop and is 3 hrs from Mumbai.
From Vapi, you take a three wheeler which charges you 100 Rs, or a taxi for 150 Rs. Most of the hotels are at Devka beach and are reasonably priced. Book them in advance if it is holiday season. You can book the rooms from Mumbai.
The beach at Devka is rocky so it is not very comfortable to walk around. There is a park there where you can relax. Booze is available very cheap everywhere.
The Jampore Beach is lovely and is a must visit. Moti Daman Fort at the mouth of Daman Ganga river a fort at Nani Daman, Hathi Park, Kachigam Water Tank are few of the places of interest. The hotels give you a list of places to see.
It is better to go from place to place by your own vehicle or a three wheeler hired for each stretch of journey within Daman. You can cross the Daman Ganga river by boat as the bridge is broken. There are boats which take you about a kilometer into the sea just for fun. It is safe. A camera is a must carry item.