OK, encrypting something.
I tend to use a 1k or larger key; sometimes many times the actual information length. This key is, of course, based on that dandy pseudo random function with either the time of day to the millisecond or the actual message as the seed.
Most of these things are rotations (remember ROT 13?) around the 256 ascii set, the number of rotations used against the key and message are many and randomly based. This allows you to play with modular math.
I used a conversion to hex mostly to allow the message (or log in, in some cases) to be sent as post (well, you try to escape a chunk of binary.)
This essentially doubles post size, but a real clever person could encrypt the hex code and throw it back into hex (rinse, repeat, etc.)
Most of this is just because it is fun and fairly easy.
Now if we were dealing with credit card numbers or socials I suppose I would have to get serious.