Well my favorite thing to use is a dropcase+sling only, but that definitely won't fly in the city.
I was originally going to go with a tennis case for my AKSU, but decided against it because I doubt anyone plays tennis in the winter around here. I went with a mandolin case instead. Obviously it's still a little shady carrying that around, but I guess it helps that I'm a musician.
I can actually fit the gun with the stock extended, a mag already in it (if SHTF), and a filled four mag pouch all at the same time without it being too tight. I don't defend my apartment with an AKSU, but it is nice to have it all ready to rock and roll if ever needed. The side pocket is huge- I keep eyes, ears, oil, cleaning gear, and first aid stuff in it at all times.
I actually stuck the NFA paperwork in the part you normally fill out that address card. It's really nice, every time I've been to the range I just flash it when asked... never was asked to take the whole thing out