These are great tips Vincenza,
I know aside from the outdoor aspect of working out (walking, running, swimming etc) you can always join a health club, or if money is an issue the YMCA or other community centers are usually really affordable.
I also recommend some indoor calisthenics i.e. jumping jacks, pushups, situps, knee lifts, running in place, and some stretching. I do this on a daily basis since I've stopped drinking and care more about my health and I get a little sweat going without actually going outside.
Definitely cheap and you don't have to worry about heat related issues.