We just returned Saturday (4/21) after being there 10 days. There were a couple of nights when we were very happy that we (well, I) packed everyone pants and a sweatshirt. Didn't need a heavy jacket but it did get windy and pretty chilly in the evenings a couple of times...the rest of the nights we were fine with shorts and a light sweatshirt.
Sometimes it's hard to tell way in advance if it's going to be chilly in the evenings, because on one of the days it had been HOT during the day so we weren't expecting it to be so cold. So I think since it's still early spring it's smart to pack at least one pair of long pants & sweatshirt of some sort just to play it safe.
Have fun!