Some things go without mentioning, like proper weighting, mask clearing. Hell mask clearing is going to come whether you want to practice or not. It's just a part of diving.
There are a few things you can do while on a dive. For instance, during your safety stop practice air sharing. Get it down and then practice "while you actually safely ascend to the surface". Sharing air is easy, maintaining buoyancy and preventing an uncontrolled ascent is what's difficult, especially if one diver doesn't have their whits. Perhaps after a talk about the planned drill with your buddy and once you both feel comfortable each of you can practice controlling both of your BCD's while air sharing to mimic a more likely scenario with one diver not being responsive to maintaining his/her buoyancy as if they were in shock and froze.
Practicing DSMB deployment with a reel or spool is another important skill, IMO. This should be done once you've nailed down your buoyancy control.
Speaking of buoyancy control. Once you're neutral, practice using your lungs to make adjustments in the water and not use your BC.
That said, there's also a couple drills I would practice in a pool first or at a relatively shallow depth, again, once you have nailed down your buoyancy control. Clearing a mask is easy once you've done it several times. Try diving without a mask. A tip I learned on here is capturing an air bubble over your eye by cupping your hand over your brow. It takes some practice, but works amazingly well once you get the hang of it and it allows you to see underwater. This is important, IMO, if you lost your mask or it failed and you also got separated from your buddy. You want to be able to see your depth gauge or computer so you can safely ascend and surface.
I've also recently been practicing ditching my gear in my pool and surfacing while keeping my airway open. Mostly to minimize the time it takes to do it. Donning and doffing to me is not that difficult to spend a lot of time practicing on, but more importantly is being able to quickly ditch your rig and surface.