What's the simplest algorithm that …

1. operates on a block of fixed size (or can be easily made to do so), i.e. the input block has a fixed size, e.g. 256 bit, and the output is a block of the same size
2. is reversible (so it can't be a hash function)
3. satisfies the strict avalanche criterion (SAC)

I'm not interested in any other cryptographic properties. It doesn't have to be a secure cipher. Name more than one if you like.