Someone with the weaponry and free time to go on a quest will probably be a young nobleman or similar. In which case going to visit older noblemen, killing them, and taking the gold they obtained in the same way in their youth would be more or less expected.
I guess the Saxons will be suitable victims in your case. In case of success be generous with gold and mead alike, that being how you get loyal followers and establish yourself as a king. Then go loot some more, loyalty has a half life, until it's time for your son to take over the looting, and other's sons to come and try to take your stuff.
Then of course there's the blood feuds that this brings about.
>but what other creatures, races, and monsters from Scandinavian folklore could exist?
Maybe these are rather later than Viking era, but I think they'll pass anyway.
Trolls and changelings have been mentioned.
The huldra, aka the skogsrå, looks like a beautiful, naked woman. Except her back is either covered in bark, or hollowed out like an old rotten tree. Either way, she tempts traveller to follow her into the woods, disappearing forever.
Näcken is somewhat similar. A beautiful young man who sits naked by the rapids of some river, playing the most amazing tune on a violin. Those entranced by the melody will walk into the river and drown.
One story has him seek out and test a wise man. Answer any question wrong and die style. When the man has answered every question correctly, the last question is "what did Oden whisper into Balder's ear at the fuenral?". Since no living soul but Oden himself knows this, the wise man failed, and was slain.
One old account of a feud have a few men come to the farm of another in order to brutally murder him. One goes up to the farm to see if he's there, and then returns to the rest, who ask if the man was home, to which he replies:
"I don't know. But his spear-axe was home, that much I'm certain of."
And then he falls over dead.