I didn't use a purse until my 30s. In college I had a backpack for class, and I wore big men's jeans so I could just put my wallet and keys in my pockets. In my 20s I had a nice small messenger bag that fit a notebook for classes, umbrella, light sweater. But the messenger bag was too heavy for an injured shoulder, so eventually I got a purse.
Now I have a very minimal leather crossbody (as a Find below) that carries my sunglasses (my eyes have real problems with California sunlight), wallet, iphone, keys, and a tiny pouch with chapstick, a few pills, and bobby pins. Some days I also carry a backpack with my laptop, a sweater, etc. On those days I also carry my purse rather than consolidating into one bag, unless I'm riding my bike, in which case the purse goes into the backpack and then I'm constantly wrestling with it to find my wallet for the subway, my sunglasses whenever I go outside, etc!
If not for the sunglasses I might try to go back to just pockets but I always end up dropping my phone when it's in a pocket so that's probably a bad idea...
For a while I had a larger, classic purse, an awesome Coach station bag I got at a consignment store. It could hold my notebook, umbrella, etc, but it was too much to carry around daily and I felt too fancy. I'm a scientist, and I'm one of the few women carrying a purse when I go to a lecture. Most people probably leave theirs in their office / desk but again, the sunglasses problem... I just can't step outside without carrying them.
I'm intrigued by the leather belt bags that are trendy this year. I could picture using something like.