As a half-day excursion from Rauma, I visited Sammal-something-something. Again: I am not able to memorize Finnish place names exceeding two syllables.
Sammallahdenmäki is a burial site dating from 1500 to 100 BCE. It was originally on the coast. But ever since the ice age ended, the land has been rising and the coast is now 15km away. The burial rites spread to Finland from other parts of Scandinavia.
The cairns are distributed along a rocky hilltop in a forest. They are easy to spot. The surrounding scenery is lovely and I agree with Els that this is probably the main selling point of the site.
There is no wall or fence and you don't have to pay any entry fee. It's surprising how relaxed the Finnish authorities handle preservation and it seems to work. Still, proper trails and some explanatory signposts would go a long way to improve the overall experience.
Getting There
As far as I know, there are no direct buses to Sammallahdenmaki. Still, with a little walking, you can easily reach the site using public transport. There are regular buses running between Rauma and Huittinen. Get off at Lappi and from there it's a little less than 4km walk to Sammallahdenmaki.
Supposedly, during summer there are also guided tours once a week (Thu 13h) from Rauma including shuttle. But I cannot find the source.
While You Are There
If you are entering via the Southern entry, you will pass a large meat processing plant. They operate a shop/restaurant if you want to grab a bite. Not sure if they cater to vegetarians, though.