8 hours in YYZ is nasty. I've had it happen twice because of weather delays.
The good news is that if the weather is fine, you can actually get out and enjoy a bit of the city. Spend a few hours at the Hockey Hall of Fame or do a tour of the Steamwhistle Brewery. Assuming that you're already checked in for your 2nd flight, I would allow 2 hours travel time, plus 30 minutes for security and 20 minutes to walk to your gate. Typical travel from downtown to the airport is only 1 hour, but an accident in the wrong place or a power failure at a set of traffic lights downtown can really ruin your day. (Summer lightening storms were the worst because they would knock out power downtown, doubling or tripling commute times to the airport.)
For spa treatment, the most convenient spa that I'm aware of is in the Sheraton Gateway Hotel (a convention hotel attached to T3). The hotel was nice a few years ago, but I suspect that it isn't as nice since Air Canada stopped using T3. Since YYZ installed a train system (replacing buses) to connect T1 and T3, it's very fast and efficient to get around. If the spa at this hotel is appealing, you should have plenty of time for a treatment. You would only have to leave enough time to get from the hotel to the terminal (2 min walking), train to T1 (5 min max), security screening (5-30 minutes), and walking to you gate (YYZ T1 is huge, allow at least 20 minutes).