Entry
Short visits need no visa for most ASEAN citizens, and for many other nationalities as well. The Immigration Department site covers what applies to your passport, along with the online arrival declaration where one is required.
Culture - take note
Brunei is a sultanate run along conservative Islamic lines, and Sharia law applies. Alcohol is not sold, though non-Muslims may carry in a modest allowance for private use, provided it is declared. Visitors should dress modestly and cover up inside mosques. Use the right hand, and point with the thumb rather than the index finger. Expect to show respect for the Sultan and for Islam. Friday prayers shut shops and offices. In daylight during Ramadan, eating, drinking and smoking in public are off limits.