If you want the easy way out, check cadomax's site (
www.cadomax.com/astuceshammerfest.php) which, though in french, has the locations of all hidden areas and items as well as a (generally) accurate depiction of items and their points values, and also how to get large crystals (if you've seen one you'll know it).
Otherwise, I recommend my hints: Drop bombs everywhere, especially anywhere that looks suspicious (IE, ledges that don't seem to have any purpose or corners away from the action). Often this is the best way to find hidden areas. For points, in the earlier levels you should almost always wait until the points items drop before killing the enemies, otherwise you're missing out on easy points. Also, if you want the highest scores, I recommend not grabbing the umbrellas unless you need them for the quest. Once the quests are finished, avoid them unless the level is hard.
I would recommend focusing on winning the game first as opposed to the highest score, partly because you get a huge bonus from beating it, but mostly because in general, the hidden areas are much harder than the main game.
The way to find large crystals is by beating enemies in a certain order. You can again look at cadomax's site to find out, or you can experiment yourself. Most of the smaller levels are easier to figure out and most of the levels with more enemies are to hard to do anyway (though a couple I've figured out on my own).
Also later you can earn an item (:item83

that will kill the enemies in the exact order they need to be killed with, and if you watch that you can figure it out.
Good luck! have fun! Have some cake!
