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You could check out the: Hershel Bag [link]
I read in the reviews that a 15.6 laptop might be a tight fit though... But it's a great bag that I've had for a little over a year.
20F, from Los Angeles, lives in MTL, and now living in France
This jacket ([link]) and a good sweater (plus gloves/hat/scarf) to layer is all I think you’ll need.
I’ve been using this type of backpack ([link]) for travel and I’ve done up to a week in it. I got a daypack at Decathalon, which has great low price outdoor gear, and fits nicely in the laptop sleeve of my bag. Also a big fan of the quick dry towel I got there too.
Otherwise, I’d recommend just bringing one big bag. Try to pack light bc bag fees are expensive in Europe!