You can use the toll roads in Orange County with a "The Toll Roads" account. Just log in and pay after using them. I think you've already found this in your research. Probably the most likely one for you to use will be CA-73 through Orange County when heading down to San Diego. However, it's only helpful if you're approaching from I-405 and not I-5.
Based on where you are traveling, the ones you cannot use without a transponder are:
CA-91 (CA-55 to CA-71)
I-110 (CA-91 to I-10)
I-10 (US-101 to I-605)
I-15 (CA-52 - CA-78)
Frankly, given the limited time, I wouldn't bother with a transponder. Sometimes the tolls can be quite pricey and don't really save all that much time. If you're on vacation, just ride with the traffic and mellow out. Or, time your trips to avoid the traffic as much as possible.