What about a less random option?
When you're out in uncharted territory looking for trouble, you find it all right. By default, it's a small group of monsters native to this region, of a level equal to the Area Rating [or whatever], who are hostile and will attack the party on sight.
Choose one of the following to be true (two, if your party is accompanied by a ranger or an otherwise skilled guide):
* They hint to or reveal something cool nearby.
* They have vital information about areas you have already visited.
* They carry particularly neat or valuable swag.
* Who said anything about monsters? You find some other sort of troubles. It is going to be trouble, though.
Also choose at least one of the following to be true. For each advantage, the GM may choose a disadvantage. You may also choose more options from the previous list, for the same cost.
* The monsters are not immediately hostile (but they won't be friendly, either).
* You lie in ambush for the monsters.
* The monsters are youngsters or weaklings, with lower level than the Area Rating.
* The monsters are natives of an adjacent region, and may not call for help or reinforcements.
* You have something the monsters want badly.
--
* The monsters are numerous, a medium group instead of small.
* The monsters are particularly fierce or cunning members of their kind.
* The monsters have a hidden ace up their sleeve.
* The monsters get the drop on you.
* The monsters are particularly adept at using that specific terrain to their advantage.
* You have something the monsters want badly.