Look around you. The world is full of traps, and you can use these to inspire traps in-game. Just the other day I was leaving an office, and as I rounded the corner I almost whacked my head against a plasma TV mounted a little too close to the door and just a little too low. Thus, I thought of the following:
A lit torch is mounted closely on each side of the door.
Type: mechanical; Perception DC 17; Disable Device 16
Trigger touch; Reset Auto or None
When a character enters the room from the non torch side, they run into the torches with their head, if they are over 5' tall. Torch(DC 17 Reflex Save or take 1 damage and becomes staggered for one round. If they fail the save by 5 or more, the torch falls out, setting the curtains on the wall on fire. In the case of fire, the trap is counted as not reset)
This is another example of a trap that isn't deliberate. Characters can face all kinds of hazardous situations, especially given that most settings don't have OSHA. You can have warehouses full of unstable boxes, crumbling furnaces, leaky water towers, you name it. Look around you everyday, and imagine that same type of setting set in the middle ages made from substandard parts and imagine what it would look like. The average town or wizard tower may just be full of dangers - and most of them could be completely accidental.
If used in the correct way, these types of traps will also lend another layer of detail to your campaign. It can turn a static, orderly, well-designed location into the chaotic, unpredictable location that will keep your players entertained and on their toes.