Mandal's old centre is a compact area of white wooden houses, narrow streets, small shops, and riverfront corners. It does not have one blockbuster monument, but it has the easy charm that makes southern Norway's coastal towns so likeable.
The best approach is unplanned. Start near the river, wander through the older streets, and let the details do the work: carved porches, painted timber, quiet courtyards, and glimpses of boats below the houses.