Is there a brush that will add either a wet look or some shine to a still life image?

Hello, With the correct blending modes, the plastic wrap filter can give some glossiness.

Likewise, an emboss layer style with some texture, and shadow set to a light color, set to screen or a lightening blending mode.

Wettness is defined by increased reflectivity and colors getting darker due to how the light gets absorbed rather than reflected. Also this perception is driven by the surroundings. A pumpkin on "light" soil will always look more dry than on "heavy", dark soil. So you may have to apply some sort of high-pass process to sharpen the highlights and/ or darken and desaturate the colors somewhere, though without at least a part of the image or a small version to have a look it will be near impossible to advise on the specifics.

Mylenium

Please post (a lores or a part of) the image. 

Without seeing the image, I'd say maybe the Dodge Tool can help.

On top of what Bojan said, you need to attempt to change the quality of light on the pumpkin. Wet surfaces have a lot of contrast in the specular highlights that act as virtual mirrors. Dry pumpkins will pretty much show just diffused highlights. You could try adding a light gray layer above the pumpkin, set the blend mode to screen, add a layer mask, filled with black, and then take a low opacity  brush and paint in harsher specular highlights. 

"I would like to add a bit of either a wet look or some shine to some of the pumpkins. "

If that means to change some colors only then use Hue/Saturation adjustments layer. Pick which colors you want to change from the second drop down list from the top then drag Hue slider and optionaly Saturation and Lightness, At the bottom of the dialogue you can refine which colors to be affected.

You can also control visibility of color changes using Layer Mask attached to Hue/Saturation adjustments layer.