You probably have a few ideas running through your head already but you now need to find a user. Users aren't necessarily going to find you so if you are a member of a local sports club, church, drama society etc. try and think how a data processing system could help them out. For example, you could keep track of the members of a choir, which sessions they attended and whether they have paid their subscription. If you know someone who runs a small business could you write a system to keep track of when staff were on duty, how much stock they have, how many sales they make? Is there an area in your school / college that could be improved? Ask your teachers, maybe they still need to write reports manually, maybe they would want a revision tool for their subject, maybe they would like web based system to book out rooms, maybe they want a program writing that could simulate projectile motion to demonstrate something in class. Once you have found your user keep hold of them, they are going to be key to you completing every part of this project.
A program that demonstrates the use of a simple encryption method such as the Caesar Cipher is unlikely to be of A-level standard due to the limited amount of processing done. However, if this were developed into a more complete system, for example with the option to use alternative types of cipher, a graphical display of the Caesar Cipher wheels, the ability to load/save data then this would be of A-level standard but unlikely to achieve a high mark for the technical solution. The incorporation of a code-cracking option that used techniques such as frequency analysis or comparing potential solutions to dictionary text to crack a code would offer access to higher marks for the technical solution, as a result of more complex algorithms being used.