Hi there,
I would suggest using the San Diego trolley as it is way cheaper than car rental, plus it is more convenient as you can just you can sit back, relax, and hop on and hop off at points of interests along the way, including the
San Diego Zoo.
Let me also mention that private tour companies also offer
trolley tours in San Diego, so if you and your wife want to learn more about San Diego, they offer a narrated tour of the Old Town, which is so much fun!
Hope this helps
Vacationbuddy