Oh, why not?
If I were...
a month, I'd be August
a day of the week, I'd be Friday.
a time of day, I'd be dusk, when one set of possibilities ends and another begins. Arguably, I'd also be dawn, except that I rarely get up that early.
a planet, I'd be Barsoom.
a sea animal, I'd be one of those weird things that lives around the sea bottom volcanic vents.
a direction, I'd north north-west, except when the wind was southerly.
a piece of furniture, I'd be a ergonomic chair that never got used.
a historical figure, I'd be much more famous and much more dead.
a liquid, I'd be a cocktail of Coke and blood. What were you expecting?
a stone, I'd be flung at an early Christian martyr.
a tree, they'd hang people on me as sacrifices to Odin Veratyr.
a bird, I'd be a raptor of some sort.
a tool, I'd be a leatherman.
a flower, I'd be the blossom on a lemon tree
a kind of weather, I'd be an ordinary Melbourne day.
a mythical creature, I'd be a dragon.
a musical instrument, I'd be a harmonica.
an animal, I'd be a jaguar.
a color, I'd be octarine.
an emotion, I'd be the sort of pleasant confusion one experiences in the early stages of the learning process.
a vegetable, I'd be unable to type answers here.
a sound, I'd be static on your radio
an element, I'd be mercury.
a car, I'd be a sculpture.
a song, I'd be "Take Me Home" by Concrete Blonde
a movie, I'd be "Into The Night"
a book, I'd be written by the all-star team of Ken Hite, Michael Marshall Smith and Robert Anton Wilson.
a food, I'd be yummy.
a place, I'd be Al Amarja.
a material, I'd be silk.
a taste, I'd be the tang of a good strong trip taking hold.
a scent, I'd be the mingled smell of rain and ozone you get during a directly-overhead thunderstorm.
a word, I'd be terribly terribly post-modern or terribly terribly biblical, neither of which much appeals.
a body part, I'd be the eyes
a facial expression, I'd be a half-smile that looked either smirking or rueful depending on the light's angle
a subject in school, I'd be history. Ignore my lessons at your peril :)
a shape, I'd be an elipse