Thanks for your feedback and I will definitely take your suggestions into consideration.
Really, the only destination that I truly, truly need a car rental for is the trip to Plitvice Lakes. The entire Istrian region (Trieste to Pula) has an extensive and flexible bus schedule.
Having a car for Bled is helpful, but it can be done using public transport.
Doing a preliminary "search" using Hertz, I entered my rental terms to be 9/9 - 9/11--renting from Koper, Slovenia (can take bus from Trieste) and ending at Ljubljana Airport. Now, the base quote given with applicable taxes is approximately 130 Euro for those three days. It "seems" reasonable, but then again I'm not sure if continuing the reservation process would then inflict additional fees.
And...yes, it is a lot of "traveling," but other than Venice, Pula and Plitvice, the only thing I have planned on this entire trip is allowing my parents to have their daily libations and my birthday celebration dinner--likely in Pula or Rovinj. They can stew in their pivo or vino while I go play. ;-)