I agree with all the suggestions made so far.
You can't go wrong at Lake O'Hara, which is about my favorite spot in the world (been there three times, always in summer; the last time, in late August, we had rain and snow one day on Opabin Plateau. Next time I'll have gloves that are better than the thin cotton ones I had that day). Opabin and Oesa and McArthur are all must-see. If weather permits, do Wiwaxy and any of the ledges ("Alpine trails") that have been suggested.
Moraine Lake is also excellent. We didn't see Lisa's pika there, but on the trail to Annette Lake we encountered a huge porcupine (who, I am happy to say, ignored us completely). In this area, you should consider the trail to Sentinel Pass, or the Annete Lake/Paradise Valley loop.
While you are in Yoho NP anyway, if you are feeling energetic and have time, the Iceline is spectacular and higher than anything else around. It is a slog to get up to the good views, but once you are there you can go for miles with great views, returning the way you came.
A very useful guide book is "Don't Waste Your Time in the Canadian Rockies" by Kathy and Craig Copeland, which is "an opinionated hiking guide". Good descriptions so it's pretty easy to tell whether you will like the same things the authors do. They rate all their hikes with footprints: 0 means "don't do"; 2 to 4 are "worthwhile", "outstanding", and "premier". I agree with them at least 80% of the time.
Another good book is Brian Patton and Bart Robinson's "The Canadian Rockies Trail Guide". Excellent descriptions, but without the rating system. These are both available through GemTrak (
www.gemtrek.com), who also have excellent maps. Or you can pick them up in bookstores in Calgary or Banff or Lake Louise.
Do have a great trip and show us some photos when you get back.