Use internal speakers with others plugged into headphones jack
Pretty sure it can't be done with the built-in headphone jack. What you could try is getting a USB soundcard (one example, but there are lots out there for $20 or so) to plug your headphones into. That should give you two options in the sound preferences.

PS, if you option-click the speaker icon in the menu bar, you get a quicker way to swap inputs/outputs than going to the sound preference pane every time.