So in my game of D&D 4e, the PCs are going to travel through the woods and the swamps, and an idea I have for this section, is that there's a type of creature in there that is more agressive than usual at this time of the year (either it is mating season, or they got driven mad due to the magic that is currently going throughout the place, I'll see). So there are going to be more risks for the PCs.
I want to specify: the creatures are not HYPER-agressive to the point where every single step taken by a PC puts them in mortal danger. It would still be possible to go through the woods and swamps and avoiding those creatures, but it would be more difficult.
Now, something I wonder is: what could attract the creatures towards the player characters? A very classic example would be shiny objects. Another example could be overreliance on arcane magic. Maybe magic could disturb them and make them more agressive. Or maybe a specific type of sound. In all cases, it needs to be something that the players can have an influence upon.
I have also planned a side quest that the players can choose to do if they want to get a guide. A guide would then help them avoid the creatures the best he can. And basically, I wanted the adventure to be tougher for them without that guide, and easier with the guide (it would still be possible to attract the attention of the creatures even with the guide, however. It's just that the risk would be decreased).
Any ideas would be welcome.