I passed through Brunei last year (August) between Sarawak and Sabah. I only had one full day there (arriving the day before and leaving the day after) which I spent at the Peradayan Forest Reserve (aka Bukit Patoi Forest Recreation Reserve). To get there, take a speedboat from the dock in BSB (by the Gerai Makan, about five minutes walk from the bus station) to the town of Bangar (aka Temburong). The speedboat is B$6 and takes about 45 minutes. In Bangar/Temburong I stayed at the Youth Centre (Pusat Belia), about 5 minutes walk from the jetty. Dorm bed B$10, has hot showers! The Info Centre is just next door and there you can organise transport to the reserve. There's no public transport here, so go in private car for B$30 return (can go as early as you like and come back after dark). All the shops are right near the Youth Centre. (Note that not many people in town will know what "Youth Centre" means, so use "Pusat Belia" when getting directions). The reserve is c.15km from town. Is really just one path, straight to the top, although there are a few old trails leading off (don't get lost or fall off the cliffs!). I had pretty bad luck even though I was there from about 6.30am till 8pm. Saw four new birds (rhino hornbill, striped wren-babbler, yellow-vented flowerpecker, grey-chested jungle-flycatcher) but nothing else that I hadn't already seen elsewhere. There are tons of birds there though, I could hear them all day, just couldn't connect with them. Would definitely be worth a few days stay I reckon. There are also gibbons, pigmy squirrels, etc.
If you need a cheap place to stay in BSB there is a similar Youth Centre there on Jalan Sungai Kianggeh, and also the KH Soon Resthouse (B$35) near the bus station (I didn't stay in either, so no guarantees on correct info)
