If you are connecting from the US you don't have to go through the extra security for the TLV flight and after a cursory customs/immigration check (channel B) you can go to the International MLL. However, if you want to go into the city, you will have to go through the full customs/immigration channel (though your checked bags will go right onto your TLV flight and not require being claimed and rechecked). But because you will already have your boarding pass, you can go right through regular security and not have to go through the special TLV channel.

THere is a Sheraton attached to T3, accessible by monorail, but not sure about day rates. I believe the MLL has showers if you really need one but no formal arrivals lounge.

You'll need Canadian coins to use the TTC ($2.75 each way; public transit) bus if you want to go into the city. The 192 leaves from pier R in the lowest level of the terminal and runs every 15 minutes to the Kipling subway station from where you head into the city. Returning allow about 1 from city centre if you are on the subway line to get back to YYZ and an hour before your flight unless you want to use the MLL. So you could have 4-5 hours of sightseeing if you wanted.