Use passive cooling by evaporation:
Soak your sun hat and wear it.
Soak one of your (hopefully) several Buffs. Wear it around your neck.
Soak another Buff, double-it, and wear it as a wrist band.
Wear long sleeves, in a light-colored top to reflect the sun.
The comment about using an umbrella (above) is spot-on. The "shade" under the umbrella could be as much as 20° (f) cooler than in the sun. That is the usual reduction when in a trees shade canopy.
HYDRATE, HYDRATE, HYDRATE!
Force drink water if need be, but do not wait until your body says it's thirsty. If you stop sweating in the sun, you have already started to suffer the effects of heat stoke. Get into shade, and soak your head and neck with water, as soon as possible.
Been there, done that, in 2015, I suffered three episodes of sudden onset syncope (fainting) from dehydration on a beautiful 25° (c) day on the Porto to Santiago route - in May. It was a clear sunny day, I failed to heed the signs of oncoming heat stroke, and woke up face down on the senda - THREE TIMES in two days. Scary stuff. But lessons learned.
Hope this helps.
Tom