Things to do at 5 a.m.
 

In November, I'm taking the megabus up from Pittsburgh (which arrives at 5 a.m.) for a 9 a.m. meeting at the University of Toronto, as it's the only thing that gets me in early enough (I would have taken the AC flight, however, it was $1075 round trip). Does anyone know anything interesting that would be open/happening/accessible that early in the morning, or if there is anywhere good to just sit and work for a while?

I'm sure Tim Hortons / Starbucks are open that early. If you just wanna sit and work somewhere, go to the lobby of the Royal York hotel - it's got very comfortable couches/chairs, , an open-space environment, and wifi.

There are secure Tim Hortons and Starbucks less than one full city block from the bus terminal.
The Toronto arrival time is only as good as a smooth border crossing for all the passengers. If someone on your bus has right to enter Canada issue be prepare for a wait at the border and a late arrival.

Not much going on at 5am, but here are a few things to consider.

Grab a shower at a nearby gym (day pass). There is a central YMCA a few blocks north of the bus terminal (two blocks N of College St). If you have meetings at 9am and have traveled all night, I suspect a shower will be a welcome start.

Timothy's, Tim Hortons, Second Cup and Starbucks are all nearby - within 2 blocks. I would recommend grabbing a bagel at one of the restaurants at Bay/College.

If the weather is nice, you can walk around Queen's Park and University, as well as the University grounds - statues and war memorials and generally not a bad place to get some fresh air.