Samoa is very religious, with most of the population following one of the Christian denominations. This means that Sunday is generally respected as a holy day and most shops and businesses are closed. You should not walk through villages on Sundays.
Many villages have a prayer curfew in place at sundown. This normally lasts around half an hour. You should be careful to avoid walking through villages at this time to avoid causing offence.
Samoan culture is governed by strict protocols and etiquette. Although allowances are made for foreigners, it is wise to avoid revealing clothing and to comply with village rules which are enforced by the village matai (chiefs), although Apia is quite relaxed in these traditions.
Women going topless is taboo, and they should only wear swimwear at the beach. Shorts should be knee length. Shirts should be worn when not at the beach. A lavalava (sarong) is nearly always acceptable attire.
Other simple things, such as removing shoes before entering a house (or, for that matter, budget accommodation), should be observed.
The main island of Upolu is known as the "modern" island, where most northern coast villages are quite relaxed with the old strict traditions, whilst Savai'i is the more traditional island, but has become more relaxed. But nude bathing is definitely taboo.