There aren't so many ways to customize your iOS 7 device without jailbreaking it. But there are some little things you can do...
First of all you can change the background and the lock screen with some nice pictures you like.
You can arrange your favorite apps. Just hold down a finger on an app until it jiggles. Then drag it to your favorite position.
You can make folders to organize your apps. Just drag an app in top of another and it creates automatically a folder for you.
You can rename the folders... also with some Emoji's.
Apps like "CocoPPa" help you to individualizes the look and feel for you iOS device.
Apps like "vCardSender2" are helping to customize your information and other contacts temporarily...
In settings - notification you can change how the notification center looks like.
You can add some shortcuts to customize your preferred typing! Settings - Keyboard
You can change the voice from Siri.
You can buy some nice covers for your device. Not really a customization for iOS but a nice way make your device unique.
If you have more ideas, how to customize an iOS device, please let me know and i will put it in this guide! Thx!!!