My favorite way to brighten up the colors in a photo is to use lab color. Out of all the methods I have tried, it seems to give the most natural appearance.
Let's start off with this picture. The green of the crazy green man seems more dull than I remember it being.
First, open your image. Go to Image>Mode and change from RGB to "Lab Color".
Next, Go to Image>Adjust>Curves. When the curves box pops up change the box where it says "lightness" to "b". This will now let you change the blue and yellow colors in your image. Grab the bottom of the diagonal line and move it to the right (like the image below). This is changing your blue. If you want more blue then move it more, if you want less then move it less. Play around to see how much blue you can get.
Now move the top part of the diagonal line to the left. This will change your yellow. I usually move it the same amount as the blue line to even out the colors, but if you have too much or too little yellow in your photo you can fix that here by moving it more or less.
You should have something similar to what is in the picture below. Don't close the box yet.
Now change the dialog box to "a". This will change your green and red colors. The green is at the bottom and the red is at the top. I typically move these bars less than the blue/yellow bars, but play around with it until you get what you like. You should have something similar to what is in the picture below. Don't close the box yet, we have one more step.
Now. to add a little contrast to Mr. Crazy Green Man we change the dialog box to "lightness". Now we will do the typical "S" curve like the one below. Click on the two spots first, then move the top one up a little and the bottom one down a little. You really only need to move them a little to give a lot of contrast. In this example I may have moved them just a little too much (it feels like too much contrast to me), but do what you like. Now you can say OK and exit out of the box.
Here is the final Mr. Crazy Green Man and his friend Crazy Yellow man. I did some unsharp max on them too to make it a little more sharp (we can talk about that another time). Make sure you go back to Image>Mode and change the color back to RGB.
Here are the two side by side. See how the colors are sharper. They look more like the colors I remember.
Here is another example:
Play around with this technique, let me know what you come up with.