I was planning to go into medicine at GCSE / A-level time (applied, got offers, changed mind) and did some work experience which I think helped, though at 16 / 17, I'm not sure I'd have been allowed to do all this at 15.
Spending time at GP surgeries during school WE time involved filing paper records, so was pretty boring. During two half terms, arranged myself, I spent a week at a local vet practice - confidentiality rules don't apply, so you can watch operations and gory bits, useful to get an idea of how you respond to that, if not the same. Another week at the local hospital pathology lab - a day each in different sections, it's mostly about how they test different body fluids and cells, interesting background, and on the last day I was allowed to watch a post-mortem - fascinating, though I think someone had misunderstood and thought I was already a medical student!
I also volunteered on a ward for at least a year, going in at weekends to chat to patients and occasionally to doctors. Didn't learn much about medicine but something about how hospitals are run and how patients experience things. I'd agree totally that 'people experience' will be most valuable, especially at this age. I suspect my teenage job of dishwashing and waitressing at weekends for three years was not irrelevant, as it showed ability to deal with people, mucky jobs, organise myself to deliver practical tasks responsively and stick with something else while studying.