There is this accessory bundle sold on Amazon that includes several useful items: 2x spare batteries, a AC/DC external battery charger (includes car adapter), LCD protector and micro-fibre cleaning cloth. All of these for just US$16.95 (plus US$3.95 for S&H within the US). It won’t link directly to the Amazon item; it should be easy enough to search for it.
The real catch is the batteries and battery charger. The batteries, in particular, are interesting. These 3rd party batteries are actually higher capacity (1600mAh) than Sony’s own original batteries (1240mAh). Or, at least, that’s what they claim.
There are other spare batteries available from other manufacturers. One other that I came across was 1300mAh. It was also in a similar bundle, going for somewhat similar price.
The standalone battery charger is very useful since Sony chose not to include one with the DSC-RX100, and instead replied on in-camera battery charging. It’s both a good and bad thing. On the one hand, there’s nothing to get lost. Even if you loose or misplace the USB power adapter, or simply that it’s not with you, you can still easily charge the DSC-RX100 from USB power sources that seem to be plentifully available nowadays. Yet on the other hand, it means you cannot charge another spare battery while your camera is being used.
The battery charger can be used with both a AC wall socket, or with a car adapter. It includes indicators to show when the battery is charging or when it has been fully charged.
Th LCD screen protector isn’t very good quality. It’s rather soft. In fact, the protector didn’t adhere well to my screen at all, and eventually I gave up and removed it.
The micro-fibre cloth is, well, just a tiny little piece of cloth. Nothing much to say about it.
The main value of the bundle is really with the batteries (there are two of them!) and the battery charger.