Al-Qadim Mosque is one of Sibu's older Muslim landmarks, standing close enough to the old centre to fit naturally into a market-and-river walk. Its value for visitors is less about spectacle and more about balance: Sibu's public face is often described through its Chinese temples and Foochow food, but the Malay and Muslim strand is just as real.
Visit from outside unless you are invited in, and dress modestly around prayer times. It is a quiet stop, but a useful one if you are trying to understand the town rather than just tick off sights.