Othman Ouahbi has a very nice example on his blog on creating automatic updating select-boxes with Ajax.
If you change the first select-box, it updates the next selectbox accordingly.
Ahsanul Bari created a nice list of links to articles on the use of Ajax in CakePHP. Some of the parts covered are:
Previously (in CakePHP 1.1) it was best to add an exit or return after you redirect method like: