Part 2: Discipline based displays.
The actual display should depend on the discipline of the power being manifested.
Clairsentience: This is a sensory and knowledge based discipline, so the displays should be as well. When using Remote View to scry into a forest, the others around the psion might smell the evergreens or hear the birds. You might use taste and feel as the Material display and the Mental display might be seeing someone out the corner of your eye when being watched with Clairvoyance, or remembering a memory that isn't your own when your party's psion is using divination based powers.
Metacreative: This discipline is something from nothing, to most, but the psion knows that the power draws on planer forces for it's creations. The most common displays are aspects of those planes; Astral ectoplasm, Etheral Mists, clinging Shadowstuff. Actually those viewing the display may influence it. Someone thinking of money might find a gold coin, actually a trinket created by his thoughts as a mental display. Of all disciplines, metacreative displays are the most unpredictable.
Psychokinesis: Mind over energy and matter. PK displays are the most violent; sparks of electricity, jets of flame, objects shattering, gravity lessening and things floating around. You might wish to change Mental displays to Energy displays for this discipline's powers.
Psychometabolic: Mind over body. Displays of this discipline may alter the psion; bulging veins and nosebleeds to changing skin & eye color and glowing hair. To others it may aggravate allergies, cause their skin to crawl, or various other physical sensations: dizziness, headaches, eyes watering, fever or minor alterations to their form.
Psychoportative: Travel and movement based discipline. The displays of these powers also give objects movement. Books might rearrange themselves, swinging lamps, dancing toys and papers scattered by a non-existant breeze. Teleportation powers could also have enviromental effects, bringing a little of the area he teleported from with him; snow, cold/warm air, scent of burning wood or water from the ocean.
Telepathy: Mind, pure and simple. All of the displays of this discipline are just a figment of your imagination. Even Material displays are illusions, you just think that something has moved or got hot. One aspect of the mind is memory and telepathic powers can trigger false memories or reawaken supressed ones.