Wood, or a window tool to "lock" the window from sliding. My old apt. had a bar that was hinged on the wall that we could slide in behind the closed window for "added" security. Same thing for the sliding glass door if you have one. Also a front door bar either on the ground or to the wall behind it depending on how the door is setup.

Checkout the Survival Forum and the Campfire for growing with the Earth Boxes on your balcony. RedFlare shows how great they are! You could grow an entire crop of tomatoes and zucchini probably!