Congrats on the Osmo Pocket! I have found these accessories to be most useful:
WiFi Adapter - This is what allows you to control it wirelessly with the Osmo phone app.
Tripod Adapter for Wifi Adapter - Curiously, the Wifi Adapter itself can't mount to anything, so this extra bit of plastic is needed. In addition to the standard 1/4" hole, it also is molded for Arca plate grip.
ND Filters - Necessary for proper shutter speeds in bright conditions. Also necessary for getting long exposure speeds in time-lapses.
Selfie Stick - I obviously don't use this for selfies, but instead for really cool low-to-ground shots (also useful for high-above-crowd shots). WiFi adapter required to get the most use out of this.
Motorized Mini Dolly - Fun! Ironically though, I wish it was even slower
4th Axis Stabilizer - If you really wanna get crazy, this guy engineers and 3D-prints a spring-tension handle that eliminates the subtle up-and-down cadence evident in most walking shots. It's pretty awesome. I did some jogging tests in the park and it looks like zip-line footage. It takes a few weeks to arrive as they are made-to-order from Australia.
I am just about finished with my NYC video, made entirely with the Osmo Pocket. It's basically a demo reel of all the types of shots you can do with it.