Indonesia's currency is the rupiah (IDR), abbreviated Rp. The rupiah's value plummeted during the 1997 economic crisis and has drifted downward ever since, and as of July 2010 you need more than Rp 9,000 to buy one US dollar. The trailing three zeros are often abbreviated with rb (ribu, thousand) or even dropped completely, and for more expensive items you will often even see jt (juta, million).
The largest banknote is the red Rp 100,000, which may only be US$10 but is still inconveniently large for most purchases. Next in the series are Rp 50,000 (blue), Rp 20,000 (green), Rp 10,000 (purple), Rp 5,000 (brown), Rp 2,000 (gray) and finally Rp 1,000.
While the new, colorful large-denomination bills are easy to tell apart, the smaller bills and pre-2004 large notes are all confusingly similar pale pastel shades of yellow, green and brown and often filthy and mangled to boot. A chronic shortage of small change — it's not unusual to get a few pieces of candy back instead of coins — has been to some extent alleviated by a new flood of plasticky aluminum coins, available in denominations of Rp 1000, Rp 500, Rp 200, Rp 100, Rp 50 and the thoroughly useless Rp 25. Older golden metallic versions are also still floating around, and you may occasionally even run into a sub-1,000 banknote. Bills printed in 1992 or earlier are no longer in circulation, but can be exchanged at banks.
US dollars are the second currency of Indonesia and will be accepted by anyone in a pinch, but are typically used as an investment and for larger purchases, not buying a bowl of noodles on the street. Many hotels quote rates in dollars, but all accept payment in rupiah. Singapore dollars are also widely accepted, especially in more touristy areas.
Banks and money exchangers are widely available on Java, Bali and Lombok, but can be a major headache anywhere else, so load up with rupiah before heading off to any outer islands. Money exchangers are very picky about bill condition, and pre-1999 dollars or any imperfect bills or (ripped, wrinkled, stained, etc) will often be rejected. Banks in general won't change any 1996 dollars.
Counterfeit US dollars are a huge problem in the country and as a result the older your dollars are, the lower the exchange rate. You will get the highest exchange rate for dollars issued in 2001 or later and the exchange rate drops for 1999 and 1996 dollars. There are even different exchange rates according to the serial number for dollars from 1996. Banks and money exchangers on outer islands are sparse and will charge commissions of 10-20% if you can find them.
In the reverse direction, money changers will be happy to turn your dirty rupiah into spiffy dollars, but the spread is often considerable (10% is not unusual). Be very careful dealing with moneychangers, who are very adept at distracting your attention during the counting process and short-changing you as a result. As a precaution, consider bringing a friend along to watch over the transaction very carefully. Be aware of moneychangers who offer great rates. They will quote you one price, and start counting stacks of Rp.20,000 notes, and ask you to count along with them. This is a ploy to confuse and shortchange you. If they realize you are onto them, they will tell you that they have to subtract 6-8% for "commission" or "taxes".
ATMs (pron. ah-teh-em) on the international Plus/Cirrus networks are common in all major Indonesian cities and tourist destinations, but may be harder to come by in the backblocks. Beware of withdrawal limits as low as Rp. 500,000 (~US$50) per day; in addition, the machines with higher limits often dispense Rp. 100,000 notes, which can be hard to break.
Be careful when using credit cards, as cloning and fraud are a major problem in Indonesia. Visa and Mastercard are widely accepted, but American Express can be problematic. At smaller operations, surcharges of 2-5% over cash are common.
Living in Indonesia is cheap, as long as you're willing to live like an Indonesian. For example, Rp 10,000 (<$1) will get you a meal on the street or two packets of kretek cigarettes or three kilometers in a taxi or three bottles of water. But as a tourist it's absolutely necessary to chaffer a minimum of 50%-70% off the initial price, otherwise you will spend your money quickly.
Fancy restaurants, hotels and the like will often slap on a 10% service charge plus 6-11% tax. This may be denoted with "+ " after the price or just written in tiny print on the bottom of the menu.
While most commercial places close on Sunday in the West, it does not apply in Indonesia, being a Muslim country. Most of them even have the largest visitors in Sunday (and national holidays) and shopping malls often become VERY crowded on Sunday. So if you plan to go to Indonesian malls and shopping centres, weekdays (Monday to Friday) is the best time to visit.
Saturdays and Sundays (as well as national holidays) are favorite days for Indonesians to go shopping and sightseeing, and as a result, most commercial points open 7 days a week. The notably exceptions are Idul-Fitri (Lebaran, end of Ramadan celebration), which most commercials close or open late up to two or three days afterwards (though most likely less applied in non-Muslim majority areas like North Sulawesi and Bali), and Indonesian independence day, the 17th of August.
To the lesser extent, the same goes with Christmas, particularly in Christian-majority population areas (North Sulawesi and parts of North Sumatera) and in Chinese-run majority commercials (like Glodok or Mangga Dua in Jakarta), as a large number of Indonesian Chinese living in major cities are Christian.
Shopping malls and commercials open at around 10 am, and street shops (and traditional markets) open as early as 6 am, and close at around 8 to 9 pm. Twenty-four hours stores (not malls) is not uncommon in major cities.