If flying the long haul from Europe to Australia / South Pacific, it’s actually quite nice to time your flights so that you have half a day or so in Brunei, which is about the right amount of time needed. Customs allow you to clear and have a wander / taxi around to see the city and come back for the final leg of the flight
For a fun half day, I’d suggest jumping in a water taxi to the small group of stilt villages just over the water on the south bank of Kampong Ayer, then head back to the centre to take a view of the Omar Ali Saifuddien Mosque and try the local sticky specialty of Ambuyat in one of the food courts of hawker stands
Not exactly a highlight of South East Asia, but it serves a purpose to break up the 24 hours on a plane