There are several accessories that I use with my Canon G16 that allow me a little more versatility with the camera.
First off I can highly recommend the flashes and triggers that can be found on this page: Flash Recommendations
Next are several things that easily connect to the body / lens:
58mm Filter Adapter: This will allow you to screw on a filter (i.e. Neutral Density / Polarizers, etc.) or the Step Up rings (see below) to the G16.
Step Up Ring Set: This will allow you to buy just one size filter (I suggesting getting a big 77mm) and use it on all (just about) the lenses you own. In this way you won’t have to buy a different filter for each lens size.