In Garibaldi Park there are many good hiking trails, but some may be quite rough and it a good pair of comfortable and sturdy walking/hiking soes would be good!
If you plan to camp make sure to bring a good tent that can take any kind of weather. Although there might be cabins/huts available in the park...it's on a first come first serve basis. So be prepared to camp if there is no room in the cabins/huts.
It's a good idea to dress in layers as the weather can change at anytime. And then there is also a difference in temperature in the valley when you start your hike and up in the mountains.
The best is to dress in layers, this way you can take some clothes off or add some clothes on!
Take a backpack that can pack alot and that's strong enough to put clothes, sleeping bags and all that in it. Even though you might only go for a day trip you should still pack enough water and food...you might need it!