Hi
you'll find an answer to pretty much any question you might have on here, so have fun looking.
In no particular order, the following might be good starting points:
-buoyancy control/weighting/trim (you'll be surprised just how much weight you can lose with practice, and how much easier the unit is to dive with correct trim)
-unit maintenance (best ways to pack scrubber/store partially used scrubber canister/calibrate head o2/silicon grease 'lubing' points/sterilising loop/etc., etc.)
-consumables (best prices/sources for batteries/sofnolime/02 compatible grease/o-rings/disinfectant etc.)
-drills (build-up that 'muscle memory'! Learn your standard drills to the point you're dreaming about them
- then learn them some more!! - search for extra drills not listed in the AP manual).
-bail-out procedures
Hope this helps.
Have fun
PH