If you're a citizen of the CDS, you may or may not be aware of the amount of public land for anyone's use. Currently, the main "public" spaces are the Church, Biergarden, and Platz of Neufreistadt, and the Forum and Ampitheatre of Colonia Nova. (The Schoss and the former Altenburg area are also public, although they may soon be revised at this writing.)
While you should feel free to use any of these locations if they are currently unoccupied, in order to facilitate use (and make sure you aren't conflicting) you can notify the Chancellor of your event, and the Chancellor (or some other member of the appropriate land-management group) can set the event for the SL events list (and provide security, if needed).
While rezzing props as needed is perfectly okay, the general guidelines implied by the covenant still hold. Putting up a 'slide display', or a chat logger, or a notecard vendor are all suitable actions, ones that substantially change the nature of the place or are unduly script or prim heavy should be cleared through the Chancellor or designated official first.
Hopefully, this will encourage more use of the public spaces!