The clothes on your back, one warm shirt, maybe a pair of tights, a very light rain jacket, and of course a credit card and ID. No racks or panniers needed, but a handlebar bag would be nice. Maybe add a tiny seat wedge for a few tools and a spare tube.

Seriously, you can get by with almost nothing if not cooking and camping. I could go with all my stuff in a jersey pocket. BTW, I am not kidding.

Of course that assumes you have water bottles on the bike and a mini pump.