I had my kids while we lived in Australia, and they were both summer babies. It was winter back in the UK that had me stumped 😂
Yes, close the windows and curtains during the day. Air the house at night while it's cooler.
Go out early. I used to hit the park at 8am when it was really hot.
Cinema...harder with an older baby, but brilliant with tiny babies. Watch film, feed baby, drink coffee 😀
Swimming, shopping malls, museums, galleries, garden centres...basically most places have air con, and most places are interesting to babies. Lots run activities too...who cares if it's you doing the museum quiz, it's always something to talk about!
Lots for inside things too. Freeze random toys in water then they can play as it melts. Painting. Foam. A load of cooked spaghetti kept mine busy for ages once, although I'm sure some will criticise the waste of food. Indoor obstacle course/assault course - just loads of pillows, cushions to roll about on.