Morning,
Congratulations on stepping up to take on one of the most challenging roles on the unit!! Let alone a Squadron of 100+ cadets - that’s awesome - and scary. All at the same time!!!
Obviously, i don’t really know your current set up, patterns and quarterly cycle, resources or staffing levels AND I speak from a smaller squadron perspective (at peak we had 70 cadets!). But, some ideas below…
First up, identify who those “floaters” are - and why. The first bit allows you to anticipate who is in this group - the why is critical as it enables you to identify the problem, how you got in this position, and avoid it happening in the future.
Having said that, it’s not entirely avoidable - most squadrons, even small ones, will end up with 1 or 2 cadets in the “floater” pool. Normally it’s the senior cadets and/or NCOs who’ve completed their classification training, or the flight NCOs - and you’re absolutely correct in that you need to keep them engaged in purposeful, meaningful activities.
Have they completed their Classification/PTS activities that are programmed in that evening? For classification, have a quick tally and see what’s gone on and how its got to this point so the same mistakes aren’t repeated. We generally only offer fixed subjects areas and run them in cycles - one year the cadets might study 3 subjects from Senior/Master (grouped in together) the following year 3 different ones. Giving them the tick in the box - but also keeping our subject delivery as tight as possible - and without cadets off doing their own thing and muddying the waters. I know a few squadrons who end up in a similar position who try and deliver ALL classification subjects - but it just makes your life harder… and your job is hard enough already!
For classification, there maybe times where cadets have already completed a subjects that has been programmed in. Perhaps they joined out of synch with their peers, or they passed it and others didn’t. Or, they flunked it on Ultifail and are now held back. Can they do a resit? Have they got SEND and needs some additional support - if so, have you got the flexibility and capacity to give them additional support? Can they be bumped up for their next classification subject, with resits in the background?
For those who have completed Classification training, can they start mentoring and supporting your instructional staff? They already know (allegedly) the subject - so can they offer support to those still going through it? Picking up skills and experience in delivery as they work towards their MOI. Sitting through a lesson you already know because you’ve already done it COULD be boring. BUT, if you reframe it, with a view to the cadet delivering this subject in the future, it could bring a new spin on things. Ask them how they would do things differently. Ask they to identify where practical activities could replace “chalk and talk” - get them to do some research on resources to help underpin and reinforce LOs. There are some vague ideas about this in the instructor guidance notes - but nothing comprehensive.
This upskilling and reframing could be applied equally across everything - from Junior Cadet training through to delivery of Master cadet training. Get the floaters to reimagine the training and see how they could improve delivery. They are closer to the target audience than most instructors; they are closer to what schools are doing now than most instructors - so harness their experiences to shape training going forwards.
Don’t be concerned about getting cadets to do activities they’ve done before. I always tried not to repeat STEM and Leadership stuff more than every 2 years. That way when cadets “resit” an activity, they bring with them 2 years of experience; they are often an NCO by that point, so bring a different perspective and outlook on a task. They might now be leading an activity, rather than being a follower - and that brings a different learning outcome for them personally.
If it’s PTS related, unfortunately, there’s not really a progressive step - moving from Blue to Bronze for example, normally needs an external course. BUT, if they’ve achieved Blue already, could they support delivery. Again, bringing their prior experiences to the fold and shaping how things are run.
Could the floaters run their own project? Or plan running their own evening? I’d imagine that with a squadron of 100, its an administrative nightmare, is there any support they could offer? Have they got any specialist knowledge or skills they could share or support with delivery on Squadron? Assuming they are older (and you have a range!) could they do a coaching course for shooting? Or CSLA to deliver sports sessions? Could they support or shadow a staff member undertaking their particular role?
There are a few standalone activities for small team activities on the RAF Stem 100 portal. They may still need a staff member to oversee them, but it could offer some insights.
Ultimately, I’d ask them what they want to do. How they want to be engaged and utilised. Find out what their skills and strengths are so you can utilise them effectively - OR - get them to identify their areas of weakness/development so that you can specifically tailor activities around this. They will/should know themselves; if they don’t, ask their peers or staff about it, and then build a bespoke package for them to help address these areas.
Hopefully that gives you some ideas!! Best of luck - and don’t be a stranger - we’re here to help!!! 
” sort of thing (done to death, as you said), talk about more interesting things relating to cybersecurity - obviously don’t teach them how to do anything they shouldn’t be doing, but make them aware of some of the things attackers could do, maybe get them to research something and present their findings. Could be things like previous notable cyber attacks like WannaCry, types of attack like phishing, and similar things.