Daman is good to plan for a one day trip. Around 160 Km from Kandivali, Mumbai. Straight and broad N-highway.
Nice roadside restaurants takes 3 hrs to reach. Best to carray a map/ list of places to visit.
Below are my comments on few places that I visited in Nov 2009:
Devaka Beach: a typical beach, not so clean, not that beautiful. You have to cross a deep inside city-market to reach. There are boats available to take you for a 10-15 minutes rides just for Rs 50 each.
Moti Daman/ Nani Daman: Two parts of Daman. There is one light house tower in Moti Daman, nice to have some snaps. There are few Banyan trees where you can take good shots on camera.
Jampore Beach: the only best place(beach). Para-gliding @ Rs 500, water scooters @ Rs 50 are available at very cheap rate. It is a commercialised beach lots of food and alcohol you can have there. Worth to visit.
Pargola Garden: just a small garden in Moti Daman. Pls do not even think about it.
Dahanu: On the way to Daman, this place can be covered. Mumbai to Dahanu is 120 Km and Bordi beach is another beautiful beach. The entire road from Highway to Bordi will give you wonderful scenery.