Using nature as an air conditioner is the best thing to rely on, assuming the temperature outdoor temperature drops below the indoor temperature. When it's cool outside in the evening and early morning hours, leave your windows open and place a big fan directly in front of the screen. If you have a window sill, box fans work really well for this. If possible, open multiple windows in your apartment to get a nice draft going. Just don't forget to close your windows when it starts to warm up again outside.
If your outdoor temperature isn't dropping much, try opening the windows and turning the fans to face out. This way, you'll at least get rid of some of the stagnant hot air in your apartment.
If you're concerned about security, it's very easy to block your window from opening enough for a person to fit through. Simply block the window track with an object, such as a wooden rod or hanger. You can leave your windows open all night and feel safe. You can also buy window stoppers that work just as well and are easy to install. It's especially important to take this extra step if you live on the ground floor of your apartment complex.