Dry mountain climate can change 40 degrees in any 24 hour period. Bring both the tank top and the parka! Seriously. It can be 90 degrees one day, a cold front roll in and be 40 degrees the next morning. In my lifetime we have had 100 degrees in August and we have had SNOW in August. Ditto for June. (Plus an infamous 1959 snowstorm on the fourth of July!) The most certain thing about the weather is that it can change.
Miscellaneous: Dress in layers when outdoors so you can warm up or cool down as needed.
Luggage and bags:
Waterproof luggage is key if you're going to be outside, otherwise, whatever suits your fancy.
Clothing/Shoes/Weather Gear: Dress light and always bring layers. IT can be hot one minute and freezing the next.
Photo Equipment: Never leave without your camera. You never know when there will be a beautiful picture waiting to be taken.
Camping/Beach/Outdoor Gear: Bring tarps for camping just in case it pours!