Apple - Keyboard shortcut for "Show Package Contents"

I suggest that you create a custom Finder shortcut in System Preferences → Keyboard → Shortcuts → App Shortcuts → +.

This way you can open packages with ⌥ as a modifier key.

• ⌘+O to open folders
• ⌥+⌘+O to open packages

If you want a single keystroke to open regular items and regular folders, but show package contents for packages (including Applications), you can turn to Automator.

Create a new Service in Automator.

The service receives files or folders in Finder.app.

The first action is Run AppleScript. Here is the script:

on run {input, parameters}
    set my_output to {}
    repeat with oneItem in input
        if package folder of (info for oneItem as alias) then
            try
                tell application "Finder" to open folder ((oneItem as text) & "Contents")
            end try
        else
            set my_output to my_output & oneItem
        end if
    end repeat
    if ((count my_output) is 0) then
        error number -128
    end if
    return my_output
end run

The second action is Open Finder Items

Save your service. I called mine BetterOpen.

Then, go to System Preferences » Keyboard » Keyboard Shortcuts » Services and assign a keystroke to the service. Unfortunately, I have not found a way to reclaim shortcuts already claimed by the Finder, so you may have to pick a keystroke other than ⌘O.

If you want to open the package in the same window, replace the line

 tell application "Finder" to open folder ((oneItem as text) & "Contents")

with

 tell application "Finder" to set target of window 1 to ((oneItem as text) & "Contents")