When my sister and I were children, we made a secret code that was totally uncrackable to people who didn't know what it was - but at the same time was ridiculously easy to read without a cipher key for people who did. First, we started with the letters of the English alphabet:
Notice that some of the letters are crossed out. In my infinite youthful wisdom, I had decided that those are redundant and therefore aren't needed. C can be represented with either K or S, Q is just KW, X is just KS, and as for Z, well, it sounds close enough to S and nobody ever uses it anyway. (Grownupy retrospective wisdom has had more to say about that since then....)
Then, we looked at each letter and asked ourselves, "What does it look like?" For example, A sort of looks like a tepee, E looks like a comb, F like a flag, G like someone sitting at a desk, H is a ladder, etc. We drew those out and tweaked them for a while, until we had:
which became the code base. Because it resembles the actual alphabet, it's remarkably easy to read - which does not make for a very good super secret code. Therefore, we decided that there should be multiple ways to write each letter.
Above is the code in full. When writing in it, we just chose the letter variations at random. It works out well because E is the most common letter of the alphabet and coincidentally also has the most variations. We did discover that Z actually is a necessary letter, and so is C because it's part of CH. In practice, we just substituted SH for every CH and got by well enough for our own purposes.
Our purposes, of course, was to annoy a mutual friend by passing notes to each other in standard teenage "ha ha! you can't read this!" fashion. In reality most of the notes just talked about ordinary things like where we should go for lunch (aka "lunsh"). The friend spent years trying to decipher it and never succeeded.
Later on I went back and added some more symbols for SH, CH, and TH.
The latter two are stolen from the Mandarin Chinese teaching alphabet. (Ever wonder how Chinese schoolchildren learn to read and write all those thousands of characters? Well, there's a cheat code...) By the time I put those in, though, we'd all grown up and moved on to other entertaining pursuits with our lives, and they were never used.
In summary, the code works on two basic secrets: