sound improvement... - any ideas?

by simple I mean easy, i.e. without6 possibility of blowing anything up. I've heard that changes the front speakers isn't a bad idea - how about changing the head unit - easy enough??

complete novice here - any suggestions aside from 'do without cd's' would be good!!!

 

Changing the head unit is a 5 minute job with the correct harness adaptor and will make a HUGE difference