Categories and Short Codes don't cut it, for the site that truly needs multiple calendars.
A simple enhancement would be to add a short code to filter which categories are made available to the website visitor. (Currently, there's a short code that filters which categories are displayed, but there is none to filter which categories are shown to the user in the drop-down for selection).
A more advanced version would allow better filtering on the admin side - so that if I am in "calendar 1", I only see those events. And when creating an event, I could select which calendar, and which category (for that calendar).