A Raincoat
It’s going to rain. I know the forecast might tell you otherwise, but it’s going to rain. When it does, I’d recommend having a raincoat in your bag, close to the top so you can get to it quickly in the event of a sudden downpour. The few extra ounces added by a packable raincoat are well worth the knowledge that you’re prepared, and saves you the deeply unpleasant experience of being rained on while waiting outside on the tarmac at a rural airport, before boarding a long, cold flight when you have to go right to work once you land.