Luggage and bags:
If you plan to stay a while, bring a bag big enough to pack for the multiple seasons.
Clothing/Shoes/Weather Gear: Spring and Fall: Think layers: make sure you've got a jacket in the very least. Evenings will get cool on you very quickly. Nobody can tell you exactly when our seasons change as they tend to do so when they so choose.
Winter: These can get very cold in Boise, as a kid we'd make igloos out of three foot snow in the valley, the past few years have been getting a bit warmer and the snow line receding back past the foothills. Snow will help insulate the city and keep it warmer so even when sliding around on the roads remember that it's helping keep your nose a bit warmer. So do bring along the mittens and ear warmers as you'll be glad that you did.
Summer: We've got a nice stretch of desert to one side so summers can get upwards of 105* F in the shade. Pack sunscreen, shorts, tank tops, etc... if you're prone to burning bring a hat. Drink plenty of water if you're out wandering around all day as the heat by afternoon will have your head spinning if you don't.
Toiletries and Medical Supplies: You can easily purchase any toiletries that you may require once in Boise at any grocery store, quick stop, etc..
Photo Equipment: Film can be had at most stores. For specialized equipment try Idaho Camera in the Yellow Pages.
Camping/Beach/Outdoor Gear: Within close vicinity to the city: (seasonal) Snow sports, hiking, mountain biking and camping, fishing, beach-like environments, etc... pack for your activities.
Miscellaneous: A local's joke for our weather:
"If you don't like the weather in Idaho wait five minutes"
I recall one day in particular driving to Meridian (a nearby town) and experienced sunshine, rain, hail, snow with big wind, and a return to sunshine!! ha!
Our city style is relaxed and the most common is jeans and a t-shirt with sneakers (tennis shoes).
Summer: hiking boots, bathing suit, wetsuit (for waterskiing), warm clothes for evenings, windbreaker/rain jacket, shorts, jeans, t-shirts. McCall is very casual.
Winter: warm clothes, jeans, ski clothes, snow shoeing clothes
Toiletries and Medical Supplies: Summer: mosquito repellant