Giants, Dwarves and Elves also play big parts - not just as PC's but as god like beings and servants of the Gods.

Kobolds are a Northern Germanic monster.  

But the main thing is their relationship with the Gods is a bit different.  They Greco-romans thought the Gods lived far away and worshiped.  The Norse Gods were a bit less inhuman.  They didn't have big temples, more like stories told about ancestors that got more and more exaggerated.  They were friendlier with their Gods.  They kind of expected to meet them.  That stranger walking through town could be Thor.  

They had druid/nymph/sylphs called Huldra

They had Fossegrimen male water spirits that played the Fiddle and were kind of Fey (not exactly evil but if you failed them they would punish you ridiculously.

Mares - Nightmarish witches that sat on you while they killed you in your dreams.

Magical Squirrels that causes havoc between an Eagle and Dragon at opposite ends of the Tree of Life.

giants and stuff

I feel like a lot of people forget about how there are giants in Norse mythology and that is another monster you could include in it.

Beowolf is old English so it might not apply depending on your interpretation. Nidhog has feathered wings with dead in the feathers. He is described as flying either before or after Ragnarok. There is a dispute about when this is supposed to happen but in the poem it's stated at the end. 

What we call giants in Norse mythology are very different from giants in DND and other mythologies. Norse 'giants' were possibly called that by Romans attempting to relate the enemies of the Norse deities to the enemies of Roman deities (titans and giants). Norse giants are more accurately called Jotun and they come in many sizes and even in different shapes. Fenir might be considered a Jotun for example. The Norse dieties and the Jotun were also related. 

DnD's  elves, dwarves, trolls, giants, and so on are very different from the Old Norse equivalents. You totally can use them for simplicity or fun and so little is actually known about what constitutes an elf in Norse mythology you don't really need to worry about them anyways. Dwarves are probably close enough in a sense as well. An important note though is that elves and dwarves might be much more similar to each other than you would think. The main differences is that dwarves are called dark elves, make things, and usually live underground. Other than that, we don't know. They could be two races of the same species even. If you want to be Norse accurate not picky, then troll is just a term for a malign magical creature/person. In the one saga, a wolf who is the mother of a evil king is a troll. Giant is a popular word to translate Jotun into, but a giant as we think and a Jotun are very different. Jotuns are all kinds of sizes and appearances from having nine heads or being super beautiful or having stone legs, heart and head. Also, almost all the major dieties have at least 1 Jotun parent so they may just be another family of the same species of whatever the Norse gods were supposed to be. The kraken is not quite in the viking era or earlier so I'm not sure if it's truly old Norse but from what little I know about them, DnDs monster fits well enough. 

Giants, but you give them illusion-themed spellcasting ability, like cloud giants but worse. You could possibly have Utgard-Loki pulling the strings as an NPC or something.