Johnston Canyon is beautiful. the "Lower Canyon" walk is very easy, and has beautiful views. Do that, and if you have the time and energy, add the Upper Falls as well. It will take another 40 minutes, but it affords beautiful views throughout the walk, not just at the end of the falls.

Bring a camera, imagination, and your creative side, and you will get wonderful images on the camera, and in your mind as well. These walks are very enjoyable and afford tremendous views throughout them.  Went in mid May, even the upper Canyon walk was not slippery. It's quite easy to do the walk yourselves, and a tour guide doesn't seem essential here. 

Enjoy this wonderful place.