At Petco, you are guaranteed at least 10 foul balls in this section every game. These are generally season ticket holders seats so it is difficult to get them, but you can get them for face value easily the day of or the day before mon-thurs games on the "ticket marketplace" at padres.com.
As for parking, I'd just pay $10 and park at the convention center, but if you are cheapo you can park for free at Qualcomm and take the trolley to the park for $3-4 total, I believe. If you are going to Qualcomm just park in Old Town or off Morena and Friars and take the trolley in. Parking at the Q is TERRIBLE when there is a game so I'd definitely recommend taking the trolley in.