Hi Hayley
Superb choice for your first JY trip - you will love it! I did a version of this trip (when it still had the Rocky Mountaineer train option) in September 2011, so if you have any specific questions, happy to help.
For starters, re your packing question, you'll find the temp varies wildly from when you are in the mountains to when you reach the coast. Take fleeces, a hat and gloves (trust me- you'll really need these on the Icefield Parkway) but you'll be in t-shirts when you are in Vancouver.
If you are trying to choose the optional excursions, you should definitely go to visit 'Cooler and Grinder' at the top of Grouse Mountain. The glacier trip is also a lot of fun and I hope you get the chance for a quick stop at Bow Lake - not all the tours stop there but, in my eyes, it was even more beautiful than Lake Louise and much less 'touristy'.
Wish I was going back again and I might just do so, but not next year as I'm eyeing up Italy or China in the brochure!
Have a wonderful time and feel free to ask if you think of any other questions
Mandy