Most applications will have one or two sidebars and often you want to control the content that should appear in the sidebar based on the action. For example you will want to show something different on the homepage as opposed to some view page. While achieving this you generally also want to avoid putting the layout into each view, as that would mean if you wanted to change the layout (e.g. put the sidebar on the left instead of the right or do some funky layout which requires an additional div tag to be added) you'd have to go through every view, which does not achieve good code re-use. It is also bad practice to have lots of if statements in column2 layout just so that you can generate the sidebar correctly.
First param in CActiveDataProvider could be a string with model name or instance of class. So, you may use CActiveRecord::cache() for cache, but you need set third param to 2, because you should cache 2 queries: counting and get data.
A simple and effective way to keep track what your users are doing within your application is to log their activities related to database modifications. You can log whenever a record was inserted, changed or deleted, and also when and by which user this was done. For a CActiveRecord Model you could use a behavior for this purpose. This way you will be able to add log functionality to ActiveRecords very easily.