Grattachecca
This shaved ice dessert, flavoured with a syrup of your choice, originated in Rome and can be found in kiosks that open up during the spring & summer months. The tell tale sound of scraping ice will lead you to them.
Going underground - Domus Aurea
An opportunity to take a guided tour within the remains of Nero's Golden Palace is not to be missed. An added advantage is that it is several degrees cooler underground.
Ketumbar
One of the many bars and restaurants set into the man made Monte Testaccio that benefit from natural air conditioning. The mountain is made up of discarded amphorae which can be seen within the walls of these establishments.
Shady spots
Rome has many gorgeous parks in which to relax. Villa Celimontana, Borghese, Doria Pamphilj and Torlonia to name a few. Choose the shade of an umbrella pine under which to while away an hour or two.
Finally don't forget the free cool water available from the many Nasone that you will find dotted around the city - very refreshing