I printed out this free pattern forever ago from Artsy Crafty Babe forever ago, and its been sitting on my desk since. I finally got my act together and whipped up one of these cute Phoebe bags and I love it!
Its got a magnetic snap so your stuff stays put. (If you need help with the snap, check out this post from Messy Jesse to walk you through it - with pictures!)
A little pocket inside for your cell phone or what have you.
It fits on your shoulder comfortably, and is roomy enough to fit your wallet, a paperback, and all of the vitals but not too massive to overwhelm you.
The pattern was simple and easy to follow - I think I may just make a few more!