It actually gets a bit hot here; being part of east county, La Mesa can heat up. But, if you jump on the highway and go about 4-5 miles down the 8 - the temp goes down 5-7 degrees. Once you hit the ocean, it's a good 10 degrees cooler.
Although, nothing gets hotter than El Cajon, unless you go to the desert! Stay away from El Cajon, the weather is terrible and it's not that nice of an area anyway.
You basically only get 2 seasons here in San Diego as a whole, no matter where you choose to live (summer & fall), it does not get frigid cold though, it's actually quite nice.
How we cope? Well, we do have a/c and if it gets really hot, we just pack up and head down the highway about 11 miles and hit the beach for the day.