Drink beer. If it’s hot, it’s probably summer, and since it’s summer there is likely a beer festival going on somewhere in or near town. Also many bars are air conditioned. All sell cold beverages.
Whine about the heat. It won’t make things cooler, but it’ll make you feel better.
If you’re smart, and abhor physical activity when it’s hot, lie in a hammock in the shade. Your Portland home probably doesn’t have AC, so it’s ridiculously hot inside. If you’re outside, 100 degrees isn’t that bad if you are lounging in a hammock under a shady tree with a cooler full of beer within reach. When it’s really hot it’s usually breezy too and humidity is rare around here (Sunday’s thunder, rain, and 80+ temperatures was a rare experience for long term Portlanders).
Take public transit if you’re going anywhere. No, seriously. Avoid traveling on foot, by bike**, or by car. The vast majority of TriMet’s fleet has pretty good AC (if that ass in the back will close the goddamn window), and the bus will probably be cool the moment you step on. Let the bus driver deal with the stressed out and overheated car drivers; that’s his job.
And if all else fails . . .
Head to the coast. It’s always cooler on the coast. It’s probably even raining. (If you’re camping on the coast, it’s definitely raining.)
**If where you’re going is down hill, by all means ride your bike! And then transport it and you back home via air conditioned TriMet.