I did it through my swimming club about a million years ago, which meant the sport was easy, and I did lifesaving and then teaching swimming as my services, which had the added bonus I could earn money with the qualifications I gained, and I was teaching some lower classes at swimming club. I still swim. And because of lifesaving, I still volunteer as a first aider at work, decades on - I just requalified last week.
My silver skill was driving, so I did something I would have done anyway, but it was definitely out of my comfort zone. Obviously that's not an option if they're still under 17, but I don't think using something you are already doing or will do is bad - you have to show improvement. There are lots of options, not just things from school.
My gold residential was related to the career i thought I was going to go into, as was the expedition project (but ultimately i haven't done that career, because of needing to pay bills...) I did my gold sport at uni (trampolining) and my gold skill when I was working - I learnt a language at evening classes. I had done all the things relying on being in a group, particularly the expedition. It was the last bit left to do, and it seemed silly to let it all go just for that last bit before I hit the upper age limit.
It can be very bonding - I'm still friends with two girls from my expedition group, and we're in our 50s now. I think the expedition can show all sorts - planning, resilience, quick thinking, dealing with adversity, team work, physical stamina and so on. I went on to do camps and hikes elsewhere in the world, which I might not have had the confidence to do had I not done DofE. None of that was strictly necessary, but it definitely contributed to who I am today. Things like scouts, guides and cadets offer similar things - the chance to try a whole range of different activities, organising yourself and your time, gaining confidence and resilience. Organisations like scouts and guides are dependent on volunteers to run it, and they also struggle because it's a lot of time for no financial reward. I am lucky I had people prepared to spend their time on me, and in turn, I do things like volunteer at a local school - partly because people did it for me.