Rauma is compact enough that a single day covers the old town, so the surrounding countryside is where a second day goes. The obvious target is Sammallahdenmäki, the Bronze Age burial-cairn site about 25 kilometres northeast — the region's second UNESCO World Heritage entry, set on a forested ridge that was a sea-cliff three thousand years ago. Allow a couple of hours for the walking trail and the drive.
The coastline immediately around Rauma rewards an unhurried afternoon: rocky Finnish shoreline, quiet swimming spots, and small harbours where you can catch boats out to the offshore islands in summer. With a car you are also within easy reach of the larger towns up and down this stretch of the Gulf of Bothnia, so a day trip can just as easily mean a museum or a market in a neighbouring town as a walk through the cairns.