If you'd like to make a menu item open in a lightbox, you can use the Lightbox2 module in tandem with the Menu Attributes module module to achieve a variety of effects.

Download and enable the Menu Attributes module.

This will let you add a rel attribute to any menu item. Add "lightframe" or one of the following rel values (as appropriate):

• lightbox for image content
• lightshow for image slideshows
• lightvideo for video content
• lightmodal for inline HTML content. Note this isn't for full web pages, just snippets of HTML content
• lightframe for everything else, including external web pages.

See here to learn how to put the node content only into a lightbox (stripping out the sidebars, etc).