How I Pack for Summer Travel in a Carry-On

'Tis the season for suitcase math.

Still haunted by a trip 11 years ago when I couldn’t fit my oversized duffel into the trunk of our rental car in Italy, thus significantly delaying the departure of our entire group, I haven’t checked a bag on a vacation since — not even for my honeymoon, which at three weeks was the longest vacation I’ve ever taken. I am and forever will be a staunch d…