While there are plenty of ideas, you really haven't given enough information for us to provide a good focused answer.

Some questions you should answer to help get a focused answer:

1. can you generate and send sufficiently securely unique keys?
2. does the user have access to your source code?
3. is the plain text "printable"?
4. need the cypher text be printable?
5. does the user need to transcribe the cypher text?
6. does the cypher text need to be self validating (include a check sum or some such)?

It may be worth noting that if you have a source of unique keys of the same length as the plain text and a secure way to transmit the keys to the recipient of the encrypted string then you can use a one time pad technique. A one time pad is absolutly unbreakable without the key and is pretty simple to implement - just xor the plain text with the key text. Xor the cypher text with the key text to get the plain text back. The down side is that the cypher text will very likely not be printable.