Night games will always be freezing. Always.
Days games tend to not be too bad. I usually go to between 5 and 10 Sunday day games a year and bring a jacket, but don't think I've ever actually put it on. It's necessary to have a jacket with you, but during the day it doesn't get as cool as people think. Tends to be around 65 and sunny. That being said, don't wear shorts unless the forecast says 70+.
I tend to gravitate towards the upper decks. Better fans up there than field level, and beautiful views of the bay (I've found that helps when the ladies aren't too interested in the game). I had split season tickets in section 313, which is upper deck down the first base line, and loved it up there. Less beer options on third deck though. But honestly, it's hard to find a bad seat in this place.
Garlic fries are pretty much part of giants games, so don't skip that.