I have been thinking this... I am quite hot by nature and wearing the binders now is already getting a bit too hot at times. I second the hiking/outdoor activities and paintballing (OH YES!), definitely brilliant ways to spend a nice day outside. But I think I will have to wear a sports bra instead

since it gets a bit too difficult to breathe when I walk a lot.
Other fun activities could be having a picnic with friends in the park or barbecuing. I am also going on holiday (at a very hot location, and no, not by choice...) so I might try to visit some cultural gems and well.. drink a little (but not at the same time)
I also know there are many exciting music festivals around this time of year so if you've got time and you love music then it's a great option too.