I'm in a rental and I installed a shower-head only filter that just screwed onto the existing shower head (and you can set the existing provided shower head aside and reattach when you move out) I found it on Amazon, it's this one "Aquasana A1-4100".
How about a hard water shower head filter?
http://www.amazon.com/Aquasana-AQ-4100-Deluxe-Adjustable-Showerhead/dp/B0016BS9T6
All about the Aquasana filtered shower head
Removes chlorine and other chemicals from your water so your body doesn't absorb it.
edit: Aquasana* fixed for the troll