I've got a Raspberry Pi 3 running 24h/24h at 100%: it is costantly at 70°C..
I'd like to know if there are simple and cheap solutions to cool it down.
There are two easy solutions:
You can combine both of course.
That temperature is the peak temperature at which Raspberry Pi can operate safely. It is recommended to use a heatsink with a fan that can be plugged into one of the pins in Raspberry Pi, as shown below
It will help it to cool down the temperature significantly. You can buy one of them at amazon or any other online store. But if you need any other more cool option, you should check out this one: https://www.youtube.com/watch?v=O7cc4eLAOMk&t=321s
Heat sinks designed for the chips work well for keeping the pi from heating up quite as fast.