Heres a quick code idea that I came up with a while back. I call it a type shift code. Basically imagine that you type as normal on a keyboard, but you shift your hands over one key on the home row. If you do this and type, everything will look like complete nonsense, but you’ve just typed in code.
For example, if I type ‘hello’ shifted one key over it is ‘jr;;p’
You can test out encoding or decoding messages here. Or test how good your typing muscle memory is.