The panel menu in the Zen Grid framework is an accordion-like jQuery menu that utilises the inbuilt Joomla core module plus a small snippet of code. It is very easy to set up once you have the correct menu structure created for your menu which can be found on the Zen Grid documentation page.

Replicating the panel menu is as simple as following and reproducing the settings as shown on the doc page. You will notice that the only thing that differentiates the panel menu display and the superfish display is the module class suffix -panelmenu. Beyond that setting up the two modules are completely the same.