I think all the practical things with day to day coping on their own, managing themselves, eating independently etc.,
It may also be a good idea to do some role playing of schools, so they have an idea of what is going to happen - have your school given you an idea of a 'usual' day?
As for knowledge, my dd started reception being able to write her name, knew most of the single letter sounds and could read simple three letter words i.e. cat, dog, can, me...etc. However, this sort of backfired, because I think she got bored when they went over them in class and switched off a bit, because by the end of Reception, she didn't know much more than she went in with and ended up on a reading recovery programme in Year 1!!!!
If I had a second child, I wouldn't bother doing anything next time, although her nursery had done a fair bit because she was so keen to do it!
She also found the noise quite hard to deal with, my fault again, we don't have the TV on much at home, she is an only child too and therefore if we did something together it was always quiet and conducive to learning, but when she got to school, there was all this racket going on and lots of distractions, she found it extremely hard to 'tune out', so maybe get them used to working with background noise.
It didn't help that they had a c@@p reception teacher, newly qualified, who had no control over the class and basically the children were running riot! (this isn't the norm so don't panic).
Letting them know what is going to happen and the routine is I think the most important thing.