A year or so ago there were a couple of threads about using field hockey bags for carrying your AR or AK on M4Carbine.net and Arfcom. Both of these threads are well worth reading if you are looking for discreet ways to carry your carbine. I think some of the examples below - though bright and colorful - are more discreet than the more tactical versions.
Other suggestions were baseball bat bags and a skateboard bag. Here are some examples:
|Bag filled with gear|
|With AR stored inside|
|Power to the AR?|
I actually have the blue "Revolt" bag with the white "Power to the People" fist shown three pictures up. As I live not too far from the hippie paradise of Asheville, I think it would fit in well around here. It is well made and it only cost $35 from Mantis Hockey Equipment.