SENSEI CAPS
This is really for Canon shooters. I HATE the Canon lens cap. If you have a lens hood on your lens and try to get the cap off you better have elf fingers. I got so frustrated with this that I started to buy Nikon lens caps for all my lenses. Since then, I discovered these little gems and purchase one with every new lens. These make me lust after Nikkor glass a little less.
UNDERWATER HOUSING
This is really a two-for-one in that I really enjoy having a point-and-shoot camera that is fully manual and takes great pictures. My G15 is an essential part of what I take with me, but the underwater housing is great for adventure trips and saved me a lot of up front coin on a housing for my DSLR. The camera and housing are relatively cheap ($399 for the G16 and $199 for the housing) compared to $1649 for an Ikelite housing. The results I’ve gotten with this cheaper combination are incredible. Side note, you can sell a camera for quite a bit more if you include the underwater housing.
CAMERA STRAP
I’m not talking about a fancy strap, just the factory included camera strap. For a long time I left the camera strap off all of my camera bodies. I felt like it got in the way and I didn’t like using it with an attached *grip. That all changed as I started experimenting with video. Now, I often sling the strap over my neck and force the camera out with both hands to anchor video shots and make hand-held pans A LOT smoother. You should give it a try!
*I abandoned my battery grip long ago because of the weight.
TREKPAK INSERTS
I think I’ve mentioned these before (here and here) but I can’t say enough good things about their product. You’ll never go back to Velcro after using a TrekPak system.
SD CARDS
Only one of my current bodies takes an SD card, but I find I use it as my default card format. Lots of people know this, but in case you didn't, SD cards are waterproof and CF cards are not. I learned this the hard way back in March when I lost a camera and lens to a wet slot canyon. No one plans on drowning their camera, but if a dry bag or underwater housing fails, it's nice to know your hard-earned pictures are safe.