We will be traveling to Croatia in May and are looking for rental car recommendations. We will be renting from May 20 - 24th, picking up in Zagreb and returning in Split. We will also be driving it to Slovenia and back into Croatia, if that makes any difference. I've checked Sixt, but they don't seem to be allowing drop off in a location other than the pick up.
Try the AutoEurope website. They're a consolidator. When you do the search for your dates and pickup/dropoff locations you will get a variety of options by car type and renal company (Sixt, Europcar, Hertz, Avis, etc). Well run and reputable. I've used them quite a bit and have had no issues with billing, cars or car agencies.
Don't forget there will be a one way fee for picking up in Zagreb and returning to Split. Also, call AutoEurope and discuss your desire to use it in Slovenia. There may be restrictions, and additional requirements in that country i.e a vignette for Slovenia.
I highly recommend contacting gemut.com, which is a good travel agency.
I always book via the consolidator Holiday Autos and have never had any issues. I tried a dummy booking for your dates with Sixt and could take from Zagreb and drop in Split. There maybe issues taking the car into Slovenia - some companies may charge more or not permit this - it may invalidate the insurance if you take it across the border without their consent.
You will need a vignette for Slovenia. Allow plenty time to cross the border.
I used UniRent for 2 weeks in September. I picked the car up in Dubrovnik and dropped it off in Split, but I see they have a Zagreb location also. So I know they allow for getting in one place and dropping in another. I also drove into both BiH and Slovenia, with appropriate paperwork (stopped and bought the vignette required for Slovenia before crossing the border).
Thank you all for the great recommendations and tips. I knew about the vignette for Slovenia, but hadn't considered border wait times!
Another question - all of the car rental companies I've looked at so far show the vehicles in terms of how many doors, how many people/seats, and how many suitcases. I find it odd that a car that will carry 5 people will usually only accommodate 2 or 3 bags. Does anyone know what they are assuming regarding size of said bags? I assume 4 carry-ons will fit in the space for 2 ginormous suitcases...
This is a good site - Rentalcars.com where you can find all major rental car providers (international and local) in Croatia. You can browse them and find on that could fit your needs.