I love it! Always wanted a secret room!
I would stash, like most others, the basic necessities. A go-bag with 5-year shelf-life water, freeze-dried food, light-sticks (no batteries needed), etc.
If you’re storing any clothing, keep in mind that sizes (for most women at least) change over time, so put in stuff that’s going to work at your skinniest (if you lost weight after disaster) and your fattest (maybe disaster happens after you’ve been binge-eating for the winter and you’re on your monthly bloat). Maybe sweatpants with ties, or pants with button tabs to loosen or tighten the waist… Don’t forget winter-weight layers, as well as socks, undies and good walking shoes you’ve actually “broken in”. And a foot care kit. And consider keeping a walking cane there in case of injury. A medkit would be useful too, with an emphasis on stopping bleeding and handling sprained/broken limbs (think athletic wrap and splinting materials).
And, of course, a backup gun (and ammo and extra magazines and range kit for the gun) properly prepped for storage (gun grease…) I might consider a revolver here, since it might be more likely to handle neglect. If there’s room and you have the resources, include a long gun as well. I would also include some junk silver coins.
I would avoid anything that needed rotation – like nuts (can go rancid) or medications or anything needing batteries. I’m thinking “out of sight, out of mind” is likely to happen here; as excited as you are about your secret room now, life happens and whatever you put there isn’t going to get regular rotation. So I would only put what can handle storage for 5 years of neglect and still be useful.