Even though some people don't love the idea of bag charms, I've always been drawn to them. To me, having a great accent for your most important accessory is a cool way to make it your own, and with all the options currently available, you don't have to go over-the-top with an outlandishly large charm. Plenty of brands offer charms that can adorn your bag or work as a key chain, and when it comes to quirky, cute options, MCM bag charms have stolen my heart.
MCM has lately made quite an impression on me when it comes to designer bags and accessories. Recently I shared the MCM Rockstar Vanity Case, which works as both what its name intends and as a handbag to carry daily. Also, I've always really loved the MCM logo, and the cute charms have won me over in all kinds of ways. These charms are small enough that they won't overpower your bag if you want to use them as a bag charm, but they also work perfectly as a keyring.
I don't own any of these yet (all the options pictured are loaned samples from MCM), and if you asked me to pick my favorite, I don't think I could answer you--they're really all too cute. A few of the charms are available at Nordstrom here, here, and here (they're having a their major sale right now so you can pick up other goodies), but every single charm is available on MCMworldwide.com and prices vary from $170 to $385.