Well autumn...
SEASONS definitely. It may be depressing, but I think the lyrics are actually more bittersweet...
Voyage would be another good one... you could try rainy day... it feels autumn to me as well.
Though most of her autumn songs are darker and more rock/experimental (talkin' 2 myself, GREEN)... I'll look more... cos it's a fun thing anyway.
And you might wanna look at the Autumn songs poll in the Ayu polls just to see what people consider her autumn songs as.
