This is a challenging question in a way, because one instinctive / visceral reaction is to say ‘what I teach is . . .’ and therefore that is what should be taught. I cannot escape that, but thought it useful to add the caveat. So, what comes to my mind, with regard to recreational / open water sidemolunt training:... for those (if any) still interested in the original topic ... what do you believe should be taught in a sidemount class at the recreational level ...
Diving in a sidemount configuration is very much about gear, so gear selection, gear rigging, gear assembly, etc. should be taught and discussed. I spend a good deal of time discussing different rigs, different cylinders, different methods of attaching cylinders to the diver, and have students try several different approaches during the OW dives. It is not as comprehensive as I would like, perhaps, but I try to show students, or at least discuss with them, a variety of gear configurations (e.g. something as simple as SPGs ‘up’ vs SPGs down, or effects of clipping AL cylinders to buttplate rails vs to a waist / hip D-ring, etc). I tell students that one of the greatest things about sidemount is the lack of restriction on how you configure your gear, and I want them to try different approaches.
Diving in any configuration is very much about mastery of buoyancy and trim, so buoyancy and trim issues associated with diving a sidemount configuration should be discussed and methods for adjusting the rig – on land and in-water - should be discussed / practiced.
Diving in any configuration is very much about gas management, so techniques for managing two independent gas supplies (unless you are diving the Z block manifold), should be taught and practiced. This is particularly important for many divers who have never used double cylinders before, or who have never entered the water with enough gas to get them into a serious deco situation.
Diving is very much about safety, and preparing for potential problems, so conducting out-of air drills while diving a sidemount configuration should be taught and practiced, detaching and re-attaching cylinders underwater should be taught and practiced, etc.
Diving is very much about adapting to different diving circumstances, in particular circumstances that necessitate different types of entries and exits, so various entry and exit procedures should be taught and practiced to the extent possible.
Diving is very much about having fun, so exercises should be enjoyable as well as instructive whenever possible – I think single cylinder diving (‘monkey diving’ is a fun activity for recreational sidemount students. I think swimming through pieces of culvert pipe, with cylinders unclipped at the bottom and positioned in front of the diver is a fun activity for recreational sidemount students. I think trying that exercise first with AL cylinders and then with steel cylinders is at least 'interesting' if not fun.
Others probably have more / better exercises that they can share.
This is not intended to be THE ONE and ONLY list, rather things that come most readily to mind.