|That guy is actually casting Mage Armor.|
Fireball, for example, doesn't have to be just a big orb of orange flame. Change up the description a bit. Maybe one Magic-User's particular Fireball just superheats the air with no real visible effects, scorching the skin and lungs of those in its area and spontaneously igniting flammable stuff. Or maybe it's a tiny green ball of fire at the center of the area that fires jets of flame unerringly at everyone in the area. If it's a Magic-User with a particular theme, go with it - how about a arachnid-themed wizard whose Fireball is a giant spider shooting flaming webs at anyone caught in the area?
How about Magic Missile, you ask? Even easier. It's not just a Magic Missile, it's Falor's Magic Missile, and it fires tiny blades to slice the victim. Or it's Allara's Arcane Shot, and it's a miniature dragon that claws at its target? Same spell effect, same mechanics, but a unique look/description. This goes for other spells as well - one Magic-Users's Stoneskin spell might give the mage the appearance of a rhinoceros, while another's might cause actual slabs of stone to grow out of the mage's skin. Again, the spell effects remain the same, but the description is all unique.
|Now that's a unique Fireball spell.|
Moving on, here's some ideas for alternate spellbooks. A specific sect of mages might inscribe their spells on faceted crystals instead of leather-bound tomes. Diabolic sorcerers may be known to carve their spells into their skin, ensuring that their "books" can't be easily stolen. Similarly, some wizards might use tattoos to record their spells (or even scrolls, if they so choose).
Similarly, magic items should be unique and interesting. A standard wooden wand is classic, but not particularly interesting. How about a "wand" that is instead a round or faceted gem that glows more brightly the more charges it has? Or a "potion" that's actually a rope infused with magic that needs to be cut rather than drunk to release the spell?
Not every mage or magic item should be outlandish and strange, however. The classics are classics for a reason, but every now and then it's good to throw in something unique, even if just to remind the players (or the DM for that matter) that magic should be magical. It's arcane, it's otherworldly power, and above all, it should be interesting.
Images from Wizards and Paizo.