Lacto fermentation is fantastic. I do it with my 4yo son, he also comes up with new things to try. We regularly do kimchi or something similar, tomatoes, green beans, red sauerkraut. He suggested strawberries which was great.

Takes about a week, cheap and very easy. A decent kimchi here is £4.50 for a small pot, maybe 250-400g? I can make 2kg+ for less. Kimchi is the only one I've seen sold out of the list too. I highly recommend trying it, if you have a vac sealer it's even easier.