First time going in Summer

Hello,
I've been to Japan during winter and spring. Now I'm planning to see what summer is like. I've read it's really hot and humid.
Where I'm from has the dry heat during summer and it cools down during the night. I'm guessing Japan will be hot all day and night.

I'm planning to go for a month in August. What is summer like in Sapporo and Tokyo?
I'm thinking of spending a week in Sapporo, then fly down to Tokyo.

Is a good idea to be travelling around, or should I stick with shopping malls or places with air con?

August can be hot, but not always. The humidity in some of the big cities will be quite different to the dry-heat you are used to. Means you might end up doing things early morning and in the evening (afternoons for siesta time)

As for travel - get out and about. We'll be travelling in July and heading north and then back down the west coast to avoid some of the city heat.

Even though summer may be hot, places like Lake Toya and Noboribetsu onsen would be cooler at night. Drops by 15-20 degrees. (Feels like)

Because one is by a lake and the other by a lake and in the mountains!

Asahikawa is significatly cooler at night too. (Not as much as the mentioned above places)