Stay in the shade if you can.
i carry a kid’s umbrella and I have a UV blocking silver one as well which folds up. I find my head gets far too hot with a hat.
im not as fat as I was, and find heat easier to bear now, but I still walk up the shady side of the street, and never stop in the middle of a sun baked plaza - figure out where you are from the comfort of the shade.
stop regularly to sip water. Bring a handkerchief to dip in the water and use it on the back of your neck if you need to.
Choose the north side in a restaurant, away from any wall that has absorbed the heat over the day. Bring a scarf to sit on as the steps / seats can get very hot
Don’t sit near the kitchens as the a/c isn’t usually on behind the scenes, and you’ll get a hot blast anytime the servers come out.
for clothes, surprisingly long sleeves stop you from getting hot. I always bring a long sleeved light cotton shirt with a collar and long skirt - it stops the sun from frying you, and is nice and blowsy.
Enjoy your trip- you’ll have a great time