Passport + CC + Toothbrush is a bit TOO minimal.
The alternative -- the bikers you sometimes see with tents, sleeping bags, multiple packs, cookware, stoves, etc etc all strapped on -- is too much for what I want.
My goal is lightest feasible load for "credit card touring"... no camping, no cooking.
If I could, I'd strip the load down to a point that it would fit in a tiny backpack I use for my daily commute. That bag holds my computer, train ticket, change of shirt and not much more.
For six weeks on the road, I decided to add a small aluminum briefcase I found on eBay. Modified my bike rack to mount it behind the seat. Checked out how much it could hold, and it was almost enough... but not quite. So I'm also going to use two front-mounted mini-panniers. Should let me balance the load a bit better around the bike, and take just a bit more stuff.
Goal at this point? Doesn't fit... don't take it.
We'll see if it is possible!