1) Playing with Levels (usually if any adjustments are done to a photo, this is my first stop). Maintaining the realism of the photo, just bumping it up a notch:
2) It's also great fun completely changing the colors & feel of a photo. It's great using filters to create "drawings" and other surreal images but also interesting to try and maintain some realism while drastically altering the photo. Again, I just played with Levels in this photo.
After playing with Levels:
Fun stuff, eh?