Two of the most important things are water and sunscreen. It amazes me every week how many adults and teenage children think they are above burning. Hello?! You can still get that beautiful summer glow even while using sunscreen.
For three of us (including the athlete) I took 6 1-liter bottles of water in addition to the athlete's larger water bottle. From 12:30-6 we went through all of it plus a can of soda. Don't underestimate how much water you can drink when you are hot and sweating. Add a little breeze and your body evaporates far more water than you realize.
Sunscreen, albeit you can still smell it and feel it, is most effective applied every 45-60 minutes. This is a theory I personally struggle with because I can still see the sheen of my sunscreen so I assume it is still working and hasn't been wiped away. And,check last year's bottle for an expiration date! It will still work, but you will need to reapply it more often because it has weakened it's strength over time.
So after remembering the sunscreen and water, there are still other perils about being outdoors. You should take snacks/food with you. If you are drinking as much water as we did, your body is going to need the salt and other nutrients from food. You can get even sicker because you drank too much water and it flushed electrolytes and other important things from your body.
We invested less than $30 for a cooler with wheels. This way we can pack perishable snacks and ice with drinks that stay cold. Ice is very heavy and can quickly deter thinking of packing a cooler. In the end, even if we destroy the wheels because of the very long gravel walk ways, I will buy one again for next season. The effort of packing and wheeling the cooler is well worth it when you feel like you are going to explode from the heat.
Also, take note of how hot you are. Being well hydrated is not the only thing that will help you from heat stroke. At the very least, a golf umbrella will help shade you from some amount of sun and likely keep you a few degrees cooler.
One last bit of advice for being outside in the hottest days of summer-if you are already not feeling well just skip long exposures to heat all together. The heat of summer causes sickness to come on quite suddenly and it can become very serious.