If you travel for any length of time, it's likely that at some point you'll encounter rain. Be prepared for it.
A lightweight or heavy-duty jacket. If you're heading into summer, just pack a lightweight shell for emergencies. But if you'll be spending a winter or a rainy season somewhere, get a really good waterproof layer. Goretex is a good choice. Consider something that has a warm inner shell and a waterproof outer shell, then you can mix and match depending on the weather.
An umbrella. Definitely not an essential, but if you plan on spending a lot of time in cities it might be worth carting one around. I like to be able to put it up and down without changing my clothes or carrying around a wet jacket (for some reason, a wet umbrella isn't as bad). We just have one between the two of us.
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