If you looking for an adventure, you can walk along the beach-side from Arambol, even if the walk is more like climbing due to the steep coastline and the difficult terrain, so don't even think about doing it with you backpack on.
A mototaxi from Arambol will cost Rs.100-200.
From the small village of Mandrem just walk towards the sea, there is little chance that you will miss it.
There is a small Portugese fort close to Mandrem, you will see it as soon as you walk out on the beach. Getting there is quite easy, free service is operated every 30 minutes but the walk is quite exhausting especially if its hot, but will give you the chance to see a sleepy small vilage were tourists still are quite an attraction. The is an abdomned school-building in the woods just shortly bevor entering the village, an almost impassable small path lead there and these 5 minutes walk are really worth it. The Fort is now turned into a High-Class hotel but the stuff dont mind if you walk around and enjoy a beautiful view over Mandrem beach and Maharashtra in the north.
Exept for this, there is little to do than laze around at the beach or take a swim in the surprisingly deep water.
Visit the beautiful deserted beach, just a few huts offering not-so-good food, but the beach and sea are worth it.
Exept for a small pan-shop at the main "bus terminal" there were no shops spotted in April 2010
Don't even think of an ATM out here so stock up on cash well in advance.
In April 2010 there were 2 small beach-shacks providing you with semi-cold drinks and a really small selection of food for reasonable prices
A few guesthouses provide you with a bed in mandrem village, but rather stay in Arambol and do a day trip because there is nothing really to do in Mandrem. But if you are looking for some quiet time one a desert beach then try bargening because there are a few guesthouses but no one really to stay in them so you migh get a better price.