Weather in Kananaskis can change quickly, and it can snow in the middle of summer at high altitudes. Dress in layers so you can peel them off as you get warmer. Even summer mornings and evenings can be quite cool.
When hiking, I find it handy to always have a windproof/water resistant jacket (great for sitting on wet ground as well), a fleece shirt or jacket, light gloves and a hat. If wearing shorts, I'll pack a pair of polyester tights, or wear a pair of microfibre pants with zip-off legs. Don't wear jeans, you'll just get sweaty and hot, or cold, and when wet, they dry very slowly. Microfibre is the best material to wear when doing outdoor activity, as it is very versatile and dries quickly when wet. It may sound crazy, but pack an umbrella when you are hiking as well; it's helpful to keep you dry during downpours, or to block the sun.
Wear a pair of runners, sturdy walking shoes or all-terrain shoes as even when not hiking, you can come across rough, wet, or muddy areas.
Toiletries and Medical Supplies: Mosquito repellant, high SPF suncreen, a pair of sunglasses, and hat. Bring water, as even though the clear lakes and streams look so tempting on a hot day, they can make you sick.
Photo Equipment: Filters can come in handy when taking pictures, because of the high altitude and subsequent strong sunlight.
Miscellaneous: If you want to have a campfire, you must bring in your own firewood. It is against the law to use deadwood in the forest, due to environmental reasons. Firewood can be purchased at Fortress Junction or in Calgary.
Make sure you have a full tank of gas, as gas is not easily available.
Duct Tape! Duct tape is handy to put on feet if your shoes are rubbing, or for general repairs, or to tie up your travel companion (just kidding). For wildlife viewing, you may want to buy a pair of binoculars. My favorite outdoor gear store is listed below.
Luggage and bags:
If you are traveling with pets then you should bring lots of water as the hikes can be very strenous and don't forget the leashes.
Clothing/Shoes/Weather Gear: Hiking boots, rain gear, socks, extra set of clothes, ball cap and sunglasses to name a few.
Toiletries and Medical Supplies: First aid kit and insect repllent are a must.
Photo Equipment: A simple camera will do, but bring extra film as you will need it.
Camping/Beach/Outdoor Gear: Towel for if you get wet.