Let's face it: not all of the pictures you take are going to turn out the way you want them, even if you have a fancy camera. This often happens while on vacation - you shoot a bunch of photos, some good and some bad. And then there are some you like, but there's just something...unappealing about the picture.
Whether the lighting is bad or washed-out, the colors are bland or the framing is off, there are easy fixes you can apply in editing mode to redeem a boring picture.
Cropping This is probably the easiest option if you have a lot of empty space, such as a washed-out sky. For example, in this landscape shot, there's a lot of space filled with water and sky, neither of which are really doing that much to help the composition.
But cropping it down makes for a nicer panoramic shot, and it focuses in on the subject without a lot of unnecessary space.
Color Enhancement Even if you're trying to preserve the naturalness of a photo, sometimes a little color adjustment is necessary. No one wants to look at a bland, washed-out photo. Some simple touches include altering the brightness/contrast, hue/saturation or shadows/highlights. I'd avoid doing ALL of these though as if you're not careful, that will definitely start to look unnatural and more like a painting than a photograph.
Borders If you have a nice subject but you're looking for an extra finishing touch, it could work well to add some details to the borders, especially if there's not much going on in the photo. For example, you can use effects like vignettes or pinhole camera style. Or for a softer, more dreamy look, try "Soft Focus" in Picasa or edge blurring in Adobe Photoshop. You can produce Diana camera-like photos without even having one.
Overlay Tones Sometimes the color in a photo is just irredeemable. No matter how much you play with the saturation or the contrast tools, it might just end up looking awful. Or having a multi-color photo doesn't do much for the photo to begin with, as in the snapshot above.
A possible remedy is scraping the color and adjusting the tone to black & white or sepia. You can always say you're going for that "antique" or "Ansel Adams" look, and these often work well for tourist pictures. On the photo above, I also added some soft focus to the edges for that old-timey look, which might not work for everyone, but I think it adds a little more texture and richness.
And if you have iPhoto, another fun one is actually named "Antique," although the tone looks more retro - like from 1970s Polaroid snapshots.
Go Abstract If you have a photo that you really want to keep (perhaps it was a photo of some object you liked), but nothing else about the photo is really working, try playing around with the special effects options in your photo software program. Maybe sampling different Filters in Photoshop or Effects in iPhoto or Picasa. Basically just play around with the features and you might happen upon something wonderful.
However, you should be wary of going overboard with these techniques if you plan on publishing your photos somewhere serious. Publishing a photo that has been considerably altered is misleading and could land you in hot water. But if its just for personal use, going on Flickr, etc., then have at it.