Besides appearing in plain view, coins can be found in locked chests. Knock those chests on your boat to open them. Each chest usually contains three coins. Coins are used to unlock paths in later levels. Floating coins can be collected by having a friendly Viking touches them. The world map occasionally splits in different directions. If there is a gate, you can only pass through by paying coins. When reaching such a junction, save your coins. Those paths offer different levels and at the end of each world is another toll gate that must be paid in order to access the last level. You can return and unlock the rest of levels later. If you save all of the Vikings but miss some coins, the level still will be marked as completed. You can return later to collect the missing coins. The coins that you already collected will be greyed out do not have to be found again.
Troll snot
The green gooey Troll snot can be used to keep objects in place. You can then cut them loose at any time.