There's a Tim Hortons on the level between Arrivals and Departures. Just take the escalator on the left past he Brontosaurus. (Or go for coffee and come back to the dinosaur and meet her there.)

Given your light baggage, you could consider the TTC. The 192 bus runs every 15-minutes from the ground level, post R. You can buy tokens from the TTC machine just by the exit door, or just pay a cash fare of $3. No transfer is required at the subway. Bus takes 10-minutes to the Kipling subway and from there it's about a half-hour to St. George (for the IC, HI or Hyatt) or Bay (for the Four Seasons or other Yorkville hotels, use the Cumberland exit). Never heard of any "independent shuttles" operating to the city, just to surrounding communities.