Letter Numbers
Replace each letter with the number of its position in the alphabet. A simple replacment method that is usually the first one taught to children and is still an effective way to obscure your message.
One of the first ciphers that kids learn is this "letter number" cipher. You replace letters with a number: A=1, B=2, C=3, etc.
When encrypting, only letters will be encoded and everything else will be left as-is. Encoded numbers can be separated by a delimiter. Numbers can also be padded with zeros so "A" is changed into "01".