A quick and easy cipher I used for personal notes as a kid, was to write in groups of three and using vowels to represent spaces between words. Vowels themselves were replaced alternately with the letters before and after them. For example the word "and" would be "znd" the first time it's written and "bnd" the next time and so on. For example:Meet me at the cemetery at dusk.would become:MDF TAM DOZ TUT HFA CDM FTD RYE BTI DTS KOOOf course it would be shockingly easy for anyone who has read the above book to crack but it fooled the teachers at my school :)V.