Running out to lunch...

Kelly Pochette or, even better, the hard-to-find Kelly Danse, which is slightly larger than the Pochette, but still fits in your 35 birkin and has a long strap you can double.
 
I like an onimaitou for quick errands. It doesn't have hardware to scratch or get scratched, and it holds a wallet (vertically) phone and blackberry. As a bonus, it has a strap and is reasonably priced (600-700). It comes in swift or box, in lots of colors.
 
Beau - the Kelly Pochette is perfect for the situation you describe and it is my personal choice for lunches. I would also suggest the Kelly Wallet but you won't get Blackberry and phone in it.

My only concern is that you are "scratch averse" and while evergrain would be fine for that use you might have issues with your hardware if you keep your KP in your Birkin.

Jige Elan.

No hardware, no issues with leather... and most importantly..... gorgeous ;)