GO Transit has a $10 one-day pass on weekends for unlimited travel on GO. And a $30 group pass for 2 people on weekdays.
https://tickets.gotransit.com/en-us/?type=promo
Personally, starting near Danforth GO, it's much cheaper than TTC with the 3-hour free return on Presto ($3.70 total compared to $6.40 on TTC - which only has a 2-hour window).