Definitely bring an umbrella -- March can be quite wet as well as the late summer storms (August), September and October's autumn showers. Layering your clothing is the key to the unpredictable temperatures during those months.
Toiletries and Medical Supplies: Bring a mosquito repellent -- your best friend during the warmer months. I'm still putting up with mosquitoes at our house during October!
Camping/Beach/Outdoor Gear: It's OK to go topless (men and women!) at the beach in Istria. But if you want to go the Full Monty, stick to the nudists (natural) beaches on the east coast of Istria.
Photo Equipment: I pause to take a picture of a departing ferry, and am accosted by a uniformed policeman. Photos forbidden! I should have known.