You can add an HDR-style effect to the image, whereby the darker areas appear brighter and the bright areas appear darker - giving your image more dynamic range. This gives you an image which looks more akin to how the human eye sees it, rather than a camera.
You can apply a mirror effect to the image, allowing you to experiment with creating symmetrical images.
Image sharpening tools emphasize the edges within an image. This can be applied to a blurred image to make it appear sharper and more detailed.
Tilt shift is a technique which can be used to achieve the “diorama” effect, whereby you can make normal images look as if they were taken of small models. It works by applying a blur to parts of the image which simulates a shallower depth of field, even on objects far away.
You can add captions and titles to your photos.
Using the brush tool you can paint across the image, or use it to make small adjustments to touch up the image.