Animals you hear and not see. (wolves howling is a good one, makes PC's nervous)
Spears stuck in the ground with skulls in the end. A candle in the skulls make the eyes glow.
The sound of wardrums in the distance (goes good with the previously mentioned spears)
A small pool that is actually a magic potion.
bizarre yet harmless animals that nature (as we know it) never intended.
Ants, hundreds and hundreds of ants crawling away from the PC's creating an open path. (if the PC's move into or in front of the swarm this becomes a monster encounter) only to disappear underground further up the trail.
Mushrooms the size of trees.
A forest of treants
Have every plant be edible or have a useful purpose.
a deer with a birds nest on its antlers, a turtle with an alchemy bush growing off its back, etc.
Puns, treant truant officers, an am-bush (a shrub that jumps out in front of you) etc.
A giant grasshopper (possibly with riders)
An elephant the size of a cat.
A group of adventurers who are the exact doubles of your PC's.
Goblins racing using chair with three wheels, the front wheel is hollow and rats run inside of it to power it, he steers the chariot with his feet. The PC's may even bet on the races.
Weird animal variations, (green rabbits, scaled mammals, feathered reptiles, furry birds, horned squirrels, winged fish, etc.)
Having a thing a PC accidentally ask for/wish for happen.
Have an NPC say, yeah right when pigs fly. Then use a space swine for an encounter.