Special thank you to Garrett Kam who resides in Bali and provided the following information.
Indonesian electric current is 220 volts, so make sure you bring a transformer for any electrical equipment. The sockets here have two round plugs, so you should bring an adaptor as well. The hotel may have a few, but with so many people coming they may be in short supply so better to bring your own.
It will be the hottest time of the year here; the rains start falling in December. So you’ll still be able to go around without fear of getting wet, although global weather patterns are out of whack so be prepared. While shorts and singlets and strapless tops are fine in the hotel, if you go outside on tours beyond the beach then more modest clothing is better. This is more the case if you go to temples, where modest wear is required. You can buy sarongs or similar material and a temple sash here rather than rent from temples at sometimes pirate rates, and use them at all temples and even on the beach.
Sandals are probably better than shoes, as they are easier to put on and remove especially if going into someone’s home. You don’t need to remove them in going into temples (there are a few exceptions, and any Buddhist one or Islamic mosque will require removal of footwear). Even rubber zori are fine, but can be a bit slippery in wet places.
In other words, dress comfortably in light cotton clothing, You can buy lots of it in Bali, and not so expensive. Even extra large sizes are available these days, especially for the BIG American tourists.
Bring a sunhat and sunglasses, and if sun-sensitive your favorite sunscreen with a high sunblock factor. The light and heat can be merciless this time of the year.
Bottled water is widely available, so do not drink any tap water (unless it’s been boiled for tea or coffee). Bottled drinks are also for sale even at small roadside refreshment stalls.
UPDATE: Another note, regarding money: Those bringing US dollars, only very new currency bills from 2003 and 2007 are accepted, and they must be in very crisp and clean condition with no marks or excessive folding on them. The best rate is for $50 and $100 bills; much less for other denominations. If you don’t want to convert too much money at one time, then $50 bills are the best bet. Most other international currencies are accepted: Singapore dollar, Malaysian ringgit, Japanese yen, Australian dollar, New Zealand dollar, Korean won, Thai baht, Taiwanese dollar, Hongkong dollar, Euro, UK pound.
If you have any more questions about getting ready for Bali or would like to add your own suggestions please feel free to leave it as a comment.