Well, okay here are my suggestions - coming from one who doesn't like schooling overly much so needs to be organised when I do!
1 Make sure you know what you want to achieve, so that you are focussed on the job rather than 'ambling'. This could be anything from achieving a good working trot, or perfect 20 metre circles, or just one clean downwards transition, or practising a dressage test. Anything and everything that you have done with your instructor, and more.
2 Then in your mind divide the schooling session into warm-up, work, cool-down. Decide before you start what each session will consist of, and do it. Don't cut corners (figuratively and literally
) if you're bored or not sure of yourself. Get in the habit of persisting.
3 Plan to use some but not all of the 'moves' you know. Circling in the corners, serpentines, 20 metre circles, turn on the forehand. Think before you school as to what you need to do to get the horses mind and body ready so that when you do these moves, you get the best result possible to you both.
4 Sometimes, get some poles out and use them to school over or around.
5 Always finish on a good note for you, and always finish with a cool down and relax for the horse. Sometimes this might mean your session is shorter or longer than you were planning. This doesn't matter, so long as you both leave the school feeling like you have achieved something.
6 If you have a video camera, see if you can set it up to film you. Even if you only get one end of the school, and just the warm-up, it will still be interesting for you to watch.
7 If you jump, put jumps up in the school and school around them. Get the horse so used to them being there that when you do jump, he's not in any way excited or spooky.
8 Do some 'de-spooking' exercises - all of which will utilise your flatwork skills. Put a sheet of tarpaulin on the ground to walk over. Find some flags to pick up and collect. Use some cones to manoeuvre around.
Let your only restriction be your imagination :)