Take a look at the Wiki on the page Mystic Forge. It will tell you where the forge is, how it is used, and some predictable recipes. The forge lets you do two things. First you can create recipes to craft other items like consumables, weapons, or armor. Second, the forge can be used to create better crafting materials of one type higher.
The examples the wiki gives are as follows:
Common Crafting Material Promotion
Common crafting materials can be promoted to a small pile of higher
tier materials with the following formula. The tier of dust needed is equivalent to the tier of the wanted product. So, if you are hoping to create Orichalcum, a tier 6 common material, you need to use Crystalline Dust, a tier 6 dust. If common crafting material B is one tier higher than common crafting material A, then:
10-50 Crafting Material B = 250 Common Crafting Material A (i.e. 250 Mithril Ores) + 1 Common Crafting Material B (i.e. 1 Orichalcum Ores) + 2-5 Dust A (i.e. 5 Pile of Crystalline Dust) + 5 Philosopher's stone
In that example: That recipe will produce 10-50 Orichalcum Ores
Armor boxes and armor satchels
Recipes can be created using the Mystic Forge. They will allow you to
craft armor boxes and armor satchels, which will in turn give you
every piece of an armor set.
Armor set recipe
Ingredients
5 Bottle of Elonian Wine
20 Arcane Crystal
50 Mystic Coin
1 Armor item
Mystic coins seem to be a common item needed for Food Trays, Weapons and Armor. Also look at this page for current recipe research.
You can see this in action with this video on how to make a Giant Chocolate Cake. Details are in the video summary.
With the exception of Southsun Cove, every zone has at least one Meta Event chain, each of which culminates in a "boss fight". Since it's an event chain, you'd have to turn up at just the right time or participate in the whole series of events preceding the boss fight to actually face the boss and get the reward chest. (Speaking of the reward chest, note that there has been a change recently to give you a guaranteed Rare or better item per account per day from boss chests, or rather, from an additional chest - see the official announcement.)
For a complete list of meta events, have a look at the wiki page, which has been updated since DavidYell's answer was posted.
Quite often, people will announce meta events on map chat (e.g. "the maw is up!"). Alternatively, there is a very nice tool to let you find out when meta events are due at Guild Wars Temple's Dragon Timer (you may wish to change the server of course).
Best Answer
As explained on the Joystiq guide to Super Adventure Box:
If someone is [...] having some serious issues getting to the checkpoint, look around near you for a mushroom sitting at an angle; shortcut mushrooms have been placed at strategic points along the map to help struggling groupmates catch up. Give it a tap and it should drop a little helper mushroom down to your buddy.The mushrooms that spawn underneath are the trampoline mushrooms that you can bounce on.