The planning session is an opportunity for the executive sponsor, ambassador user and developers to contribute together to build the project map and timeline.
The following describes a variation of "planning game" from XP that I like to use. I call it Blitz Planning to emphasize that it goes fast. However, I also like to refer to it on occasion as a Project Planning "Jam Session" to emphasize that everyone is supposed to play together and off each other in a collaborative way, as in a jazz jam session. If you lose the amicable collaboration, you can do the technique, but you lose many opportunities to creatively improve the project plan.
I find this technique works well for a planning horizon up to about three months. After that, the amount of detail is overwhelming. For time horizons longer than three months, I use the Project Map described among the work products.