La Castile Steakhouse, Alioli Ristorante both in Mississauga have a patio and both have amazing food I highly recommend both.
If you can travel a bit go to Spencer's At The Waterfront in Burlington it has a great view of the Toronto skyline from the patio on a Clear day or night and the food is very good as well.
Snug Harbour is also another good spot in Mississauga it's right on the water.
La Castile is a great choice, but if you are looking for more casual, The Wild Goose at Eglinton/Renforth has a good patio and friendly service. Arizona on Carlingview across from YYZ has a smallish patio that is best in the afternoon.
