Hoi An sits on a peninsula among a cluster of islands on a wide river delta. The main river branch passing through town is called the Thu Bon, but seems to also go by the name Cua Dai estuary. As you're facing south from the Japanese Bridge, you're looking at another parallel peninsula, An Hoi Islet, which is now bursting with hotels, restaurants, spas and a night market, where they have a whole area devoted to just silk lanterns. Beyond that, visible to the east, is Cam Nam Island, and beyond that, to the west, is Cam Kim Island. Cam Nam is accessible by bridge or boat, and Cam Kim is accessible only by boat.
The heritage town is roughly within the area south of Phan Chu Trinh, west of Hoang Dieu (which connects to Cam Nam Bridge), and extending east past the Japanese Bridge along Nguyen Thai Minh Khai Street until it reaches the roundabout. The southern border is the river. The heritage town is closed off to motorised vehicles daily 08:00-11:00, 14:00-16:00 and 18:00-21:00. Even if you rent a motorbike you may find yourself needing a bicycle to get around the area -- these are widely available for 20,000 VND a day.
Keep in mind that due to it occupying low ground and being surrounded by rivers, mountains and the ocean, Hoi An floods every rainy season between late October and mid-December, at least on some days. Some years it's only knee-high waters on a handful of days, but in other cases the waters have risen up to two metres high and have, along with mud slides in the nearby mountains, been severe enough for the whole region to be classified as a disaster area. If going during wet season, be sure to check the weather (Foreca.com is a good resource) during the days leading up to your departure to ensure no typhoons are on the way. If a typhoon is heading to central Vietnam, be prepared to change your plans; Mui Ne in the south and Sapa up north are both lovely at this time of year.
If you do make it to Hoi An in wet season, it's wise to check into a hotel that occupies high ground -- the guesthouses north of Ly Thuong Kiet, and most of the accommodation near both Cua Dai and An Bang beaches are safe bets. The entire heritage town is prone to flooding, so forget those romantic riverside retreats. If for some reason you do stay in the flood zone, be sure to request a room above the ground floor. Travel in and out of Hoi An is generally not affected unless there's a typhoon, but many businesses shut their doors during floods so think twice before prepaying for that tailored suit or dress, or booking a cooking class. Bear in mind too that the more typical floods are nothing to be too frightened about -- the waters usually recede almost as quickly as they arrive, and no matter where you are, high ground is never more than a few hundred metres away.
ATMs are widely available (they really want you to cash up and spend here!) and charge around 20,000 VND per withdrawal, with most having a withdrawal limit of 2 million VND -- however Agri Bank on the corner of Tran Hung Dao where it meets Le Loi has a withdrawal limit of 5 million VND and does not charge an ATM transaction fee. Most of your banking needs can be met by heading to the intersection of Tran Hung Dao and Hai Ba Trung Streets. To the north of the intersection on the left is the Vietcom Bank, which has no ATM but cashes travellers cheques for dong for 2% commission and offer credit card advances at 3.3%. They also receive, but do not send, Moneygrams. The TechCom Bank is north of the intersection, kitty-corner to the Vinh Hung 2 Hotel. They have a 24-hour ATM, do not cash travellers cheques at all, and offer credit card advances at 3%. Western Union is available here. Sacombank is on Tran Hung Dao, to the east of the intersection, next to Treat's Same-Same But Different Cafe. Staff here speak excellent English, so if you have a complicated transaction head here. They cash travellers cheques for 2% commission or US$2 per transaction, whichever is more, credit card advances are available at 3% commission and they also send and receive moneygrams They have a 24-hour ATM that charges a withdrawal fee of 30,000 VND for each transaction, with a withdrawal limit of 2 million VND.
Internet is available everywhere, though rates vary widely and it may save you a couple of dong to find a place that doesn't cater to tourists -- bear in mind though that most local internet cafes are predominantly set up for gaming so if you need to use Skype you will struggle over the noise. Most hotels make free internet terminals available in the lobby to their guests only. Hoi An has now got Mega Wifi, which you can access everywhere -- even as far as Cua Dai beach -- and it's free. Power outages are frequent and connections can be spotty.
The main post office is on Tran Hung Dao Street at the corner of Hoang Dieu. It's got a nice interior -- take a look up at the wood carvings in the ceilings -- and provides chilly air-con and nice desks to sit at and write postcards. There are several old fashioned wooden phone booths from which you can make a private, but expensive call home.
There are several ticket stalls run by the Hoi An Office of Tourist Services where you can buy tickets for the Culture Tour on the corner of Le Loi and Tran Phu, on the corner of Hai Ba Trung and Than Phu, and on Tran Phu between Hoang Van Thu and Tran Quy Cap. Tickets allow entry to five of the 21 ancient sites and are valid for the length of your stay. A good tip is to hire a private guide at the ticket booth, which costs an extra 70,000 VND on the 120,000 VND ticket price. Guides who speak English, French, Vietnamese and Russian are available.
Hoi An's main hospital is at the intersection by the traffic lights where Le Loi meets Tran Hung Dao, but private Pacific Hospital is where tourists usually head, on Phan Dinh Phung Street. For non-emergency medical attention the English-speaking general practitioner on Le Loi Street is a good (and far cheaper) port of call.
Most boats for Cham Island depart from the Coast Guard Pier, known as such because there's a Coast Guard office there. All visitors to the island must buy a permit, which is included in the ticket price for tourist boats but can be arranged at the office if you plan on taking the local supply boat. Remember to take your passport if you wish to stay overnight. Once you take Cua Dai Road to the beach, it's about five kilometres south (look out for the huge lighthouse). But in all likelihood, you'll be driven there.