Mountain weather is extremely moody so always carry layers.
If you've ever been in the mountains, you know that the weather can change at any moment. It could be a clear day in town, but in the mountains, it could be pouring rain. It's unpredictable which is why we always recommend carrying a rain jacket, an extra fleece, and waterproof shoes as backup. Be prepared for anything.
Also a lot of hikes and viewpoints take you to glacier lakes and high elevations which can make things cold. Make sure you carry a sweater or jacket to keep you warm so you can enjoy the views.
At night, the temperature can drop significantly so be sure to have a jacket handy as it'll come useful and the sun starts to set.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.