Depends of you age and interests but two altenatives are free outside shows one of them from the famous Cirque du Soleil

http://www.quebecregion.com/en/what-to-do/ideas/cirque-soleil?a=vis

The second one is the Quebec City History Retrospective , an outside projection on the " World's Largest Projection Screen" i am not sure that you may have an english version of the audio, but the projection is impressive and it is free if you dont have reserved seat, so no risk.

http://www.quebecregion.com/en/what-to-do/ideas/image-mill?a=vis

One of the most popular activities in Old Quebec (where the Chateau is located) is simply wandering the charming cobblestone streets of this unique Historic area (both the "Upper", and smaller winding alleys of the "Lower" City), and enjoying the architecture, the cafes, bistros, art shops, street artists and musicians, etc. When you say your daytime hours are committed I'm assuming that means you will not have time to do this during the day? (i.e. are you committed with indoor meetings?). If that is the case, you would probably enjoy spending at least a couple of your free evenings doing what I described.

An Addendum/Correction to my last post - I get confused with the "Chateau" hotels - I was thinking of the Fairmont Chateau Frontenac which is in the heart of Old Quebec. Adding to the confusion there is a Fairmont Chateau Laurier in downtown Ottawa (and I have a tendency to mix up the 2 names in my mind). But I believe the Chateau Laurier that you are referring to is just outside of the Old Quebec (Vieux Quebec) district of Quebec City. However other than the fact that you will have to walk a couple of extra blocks, the above would still apply.

Thanks to all

I agree strolling around the Old City is fun in the evening as well as during the day. there are quite a few street musicians.

There are also evening tours--many seem to concentrate on ghost stories.