Yep - there’s load of options if you don’t want to fit or can’t fit panniers.
I used one of these when we went to Luxembourg, fit enough for 12 days in the large one. Didn’t use half the stuff I took like tights n that:
(Just check you’ll have enough space between top of rear wheel and saddle for it. You normally would unless riding a small bike).
There’s also Carradice bags and things like this:
https://www.alpkit.com/products/koala