Take a boat from one of the many harbours and ports in and around Jakarta and you will inevitably end up going past small islands full of green trees front by yellow or white sand. These islands are know as the Thousand Islands or Pulau Seribu. However there are not a thousand only from around 110 (wikipedia) to 128 (Thousand Islands website), so who knows and to be honest it does not really matter. The Thousand Islands are little places of escape for Jakartans and hopeful travellers looking to get out of the big Durian for a while and slow the pace of life down through sun bathing, different water sports, snorkeling and swimming or just plain nothing.
Getting to the islands can be as enjoyable as your pocket is deep. There are chartered ferries, public ferries, fast boats, fairly fast boats. With some islands only 20 minutes away and others more than 3 hours, it does depend how you want to get there.
If you want the thrill of a fast boat and have not desire for crowds and sweat then head to Ancol and take a boat from there. If you want a ferry then get to the Fishing port at Muara Angke, next to Pluit and negoiate a ticket and clamber on board a large wooden boat with no seats, just the floor to lie or sit on and enjoy the sea trip.
Recently I have been to Pulau Harapan in the East of the islands and 3 hours by ferry or and hour by fast boat, but thats a blog to come.
Here is a map. Which does not name all the Islands. Harapan is above P.Kelapa island or Coconut Island.
Prices start from 30,000 rupiah one way to well what ever. Just remember, that the more you pay, the 'quicker' or more comfortable the trip. Safety, no comment, its best not too. However, if you have ever been to an Asian Bus Station, Airport, or harbour before then the cheap boats are just like that hot and sweaty crowded moment you enjoyed then but maybe a little more...
Couple more links for you