Dear David:
There is an advanced add-on model of the ISS containing a detailed external docking port, which is used by the Space Shuttle. In Celestia, you can load that ISS model, go to it, locate the docking port, and using the steering keys on the numeric keyboard, and the keyboard speed controls, (the "A" key, "Z" key and "S" key), you can try your hand at docking with ISS. It is difficult, because it is moving. You have to come in at an overcompensated angle, then adjust as you close on the model.
To see a few screenshots and get the advanced add-on of the ISS, go to the Celestiamotherlode here:
http://www.celestiamotherlode.net/catalog/show_addon_details.php?addon_id=293
Follow the instructions for installing this add-on. It is a bit complicated, since the authors assume you have some initial knowledge of ssc files and so forth. If you are completely lost on how to install the ISS into your Celestia folder, post a new message here and I'll try to walk you through it.
With respect to flying through Saturn's rings, that's simple. Launch Celestia, then click this link:
cel://Follow/Sol:Saturn/2003-08-13T18:24:36.33438?x=sAT/7pYn4oSyDA&y=Cl/BDLi96/n9/////////w&z=NW4Dd9b/aL9x/////////w&ow=-0.834022&ox=-0.259538&oy=-0.480674&oz=0.077460&select=Sol:Saturn&fov=34.659473&ts=1.000000<d=0&rf=40855&lm=0
You will be positioned outside of the rings, ready to start your engines and fly through them (aim for the Cassini Division gap). Use the throttle keys and steering keys. Make sure you press the [S] key to stop your ship when done.
Enjoy
Frank