Wear sun glasses, sunrays are damaging to the eyes. Its worth investing in a good pair which have a UV filter and are polarized.
A hat with a brim is suitable. If you are at a very hot place, get a hat which has a brim at the back so that the nape of your neck is covered.
Take along clothing which protect you from the sun’s raise. Eg: long sleeved shirts and light trousers.
Always take a jersey with you.
Dressing in layers is a good idea.
Wet wipes are useful.