Mystic Muffin is one of my favourite cheap eat restaurants in the city! I love their breakfast bagel deals (the $5 ham, egg and cheese fills me up right to lunchtime) and the owner is a super nice guy. I come here on a regular basis on my way to work and have never been disappointed.
In terms of lunch dishes, there are a couple of sandwich options (falafel, chicken salad) and either chilli or a soup option. There is also a salad bar and the prices are again pretty reasonable. Everything is tasty but basic, so don't expect it to knock your socks off.
There is plenty of seating inside and the owner's $5 banana loaf is perfect to bring back to work and share with your colleagues. Definitely not the best lunch you'll ever have, but the prices are very reasonable and the service is so good you'll keep coming back!
Make sure you let the owner know if it's your first visit, he will offer you a free slice of apple cake to take with you!