Work under an awning or umbrella.
Wear a baseball cap, cowboy hat, or other cap with a brim, pulled low to put your eyes in shadow.
Wear polarized sunglasses.
Equip your laptop with an anti-glare screen. (The last time I tried this, a few years back, I couldn’t actually find any that worked, but perhaps the technology has improved.)
Equip your laptop with a sunshade. You can buy a commercial version or spend a few minutes with a big piece of cardboard and some tape to make your own.
Take frequent breaks to look away from the screen to ease your eyestrain.
Give up, have a beer, and decide that the summer sun is more important than work. You probably need to stop billing in this case, though.