Dress up as Ragnar and Lagertha or how about Rollo in his French fatigues or Lancelot and Guinevere. There are plenty of medieval re-enactment societies around who may be happy to assist or even participate. This party would work well with a spit roast and lots of wine and lager.
Play pin the sword on the viking or what about Ragnarok throwing (use a big rock) – try your hand at the classic Viking game Koob which is a great game for from 2 to 12 players. You can buy a ready made kit or just cut the blocks from a piece of 2″ square pine. Get the rules and instructions from here.
There are also some great fun Nerf like X-Bows or try one legged jousting but don’t forget to use well padded jousting sticks.