I've been caught without a book, a notebook, tissues... when I've really needed them. Nope, they're in the other bag, at home. If I could, I would cast an undetectable extension charm on any bag I choose and then I could simply bring the entire lot with me. (My imagination is sometimes bigger than the bags I carry.)
And because my bags to tend to be on the larger size, I am constantly rummaging around to find things. That's when I ducked into Charming Charlies one day and picked up a very inexpensive makeup bag. I use it to hold my earbuds, Ricola lozenges, more pens, nail clippers and file... Then I thought, hey, why don't I use one of these for my receipts. I bought another pouch a few weeks later. Then one for my iPad keyboard. Another for my writing supplies (one of my favorites), and on it went.
Old-fashioned pencil cases and slim pouches in all kinds of materials, originally made to hold tablets and other office supplies, are now being used like a purse within a purse. Just grab and go.
And the purse companies are finally catching on. For me, you can't have enough pockets inside, both opened and closed, but this one has the beloved pouch right inside. And this company makes some beautiful bags, including a few pouches of their own. I like this one and this one.
P.S. You may also like A Little Reminder.