One way to get inspiration for a trap is to look through the list of spells available to spell casters. Pick one, and then try using it in an interesting way, and perhaps tweak it a little bit to fit the effect you are going for. Take the following trap for example, which is based on Antilife Shell.
The cliff edge overlooks a placid lake, dozens of feet down.
Type: magical; Perception DC 31; Disable Device 31
Trigger touch; Reset None
The lake below is cold and deep. It has also been covered with a type of antilife shell. Non-living stuff passes through it just fine, however, when someone jumps... Lake Barrier (6d6 falling damage)
It's hard to get players to walk into traps. You could use a wall of force here, but a clever player will throw down a rock or do something else to discover it. Unless the players use detect magic or something, they probably won't notice this. Add some kind of "pressure" element at the top, and the first one that jumps will be in for quite a surprise.
However, the true payoff for this trap, whether is damages players or not will be the next time they come across a similar setup. Perhaps the mage will start carrying around a satchel of gerbils to toss of the cliff before descending. These are the types of experiences that make adventures truly memorable. "Hey, remember that time when you summoned a dire rat in mid-air just to check to see if there was an anti-life shell?"