what can I grow under my spruce tree?

The problem with trying to grow something (including grass) under a spruce tree, is not the acidic needles (they don't affect the soil much and Calgary's soil is alkaline anyway), but that it is dry, dry, dry.  Not only do the branches prevent much rain from reaching the ground, but the extensive root systems suck up all the water around the tree. 

In my opinion, the best thing to do is to put down some mulch (bark chips or shredded bark), and let the cones and needles fall where they may.  Don't let someone tell you to plant moisture-loving shade plants such as hostas and ferns under a spruce - you will be doomed to failure (sorry Melanie and Megan, if you're reading this)!!  If you really want to try to grow something (good luck and be prepared to water), try something that is extremely drought tolerant:

for sun:

• snow-in-summer

for shade:

• epimedium (also called barrenwort and bishop's hat)
• lamiastrum
• goutweed (but beware it will spread invasively if it is not contained)
• lily of the valley (same caveat as above)