I have a 6 hour layover in Zagreb in April. Is the airport near anything?
The airport is small, i.e., things go quickly and smoothly because there aren't crowds. There is an easy bus connection to Zagreb. The bus station, next to the train station, is within easy walking distance of museums, public buildings, restaurants, etc. I don't remember if there are lockers where you can store luggage in the airport, but you can check on that.
To piggyback off Anita's comment, there aren't any lockers at the airport.If you do decide to head into town, I know I've used lockers at the train station and I'm nearly positive there are also lockers at the bus station too.
And to add one more thing, if you decide to take a cab to/from the airport, make sure to ask the driver first approximately how much the fare will be. Even better if you ask a few cabbies and take an average, as Zagreb cabbies can be, well, shifty.
Are the lockers at the train station fairly large? We might stop by for a few hours on our way back from Plitvice and we need to stow our luggage. it looks like Zagreb is an interesting and fun stop!
Most luggage lockers can handle a large suitcase (lockers are sometimes available smaller also). We had six hours between the bus arrival (from Plivitce) until our train to Lublijana - took the tram between the bus station and train station, then wandered the old town from there (after stowing backpacks in a locker). It was easy to see in a six hour layover (didn't fly in, so don't know those dynamics)