The factory stereo in the RAV4 is generally considered to be mediocre 
at best.  It includes an ordinary Toyota head unit (the dashboard piece), 
and cheap paper cone speakers.  The best recommendation is to replace 
the speakers and the head unit.  Beyond that, more speakers, including 
a subwoofer, could be added; also a separate tuner and amplifier can help, 
as can upgrading the wiring.  You can also add a CD player/changer; many 
people have a CD changer mounted under the driver's seat, or have a small 
changer mounted in the dash in the "cubbyhole" space.  If budget is tight, 
start with replacement speakers, then upgrade the head unit, then do the 
other add-ons.

Ken Lee's RAV4 Page has some good pictures of his stereo upgrade 
installation, plus pointers to step-by-step instructions for speaker 
and head unit replacement:

<http://www.rahul.net/kenton/rav4/>

For general car audio information, check the rec.audio.car FAQ:

<http://www.mobileaudio.com/rac-faq/>