As GM, would you like a surprise every so often "My players already surprise me all the time!". I mean the game world surprising you, just much like it surprises the players?
The idea for the rules is pretty simple, it's simply a matter of adding a bit more treasure (or gear, whatever you use in your RPG of choice). The treasure, for whatever reason (magical or mundane), after being picked up, has to be used that day and only works for one encounter. The treasure is a kind of weapon, always giving better bonuses than usual stuff - your rolling on a chart (say a D20), so if you roll high (say a nat 20) its pretty damn awesome gear (but remember, only works for one day or one encounter, whichever comes first - this isn't permanent weaponry). Make everything else on the list a bit varied, not just meleee weapons, some more powerful ranged ammo or such as well.
The thing is, the players secretly roll on the chart. Ideally you as GM go to the toilet or something when they do this, so they can discuss it. Because the idea is they are not to mention the item within your hearing. That way you don't know if they got the super kick ass item or whatever.
That way when/if it comes out (ie, perhaps just when the players are getting wooped by a monster), its a surprise for you as well! Suddenly out of the blue an awesome weapon appears!
Maybe that doesn't seem like a feature if you want to be able to see all game world events coming in advance...
If you want to be caught by surprise a little bit, though, that's one method of doing it!