...Oil? well, I'd leave it for the scheduled change. I can't see any advantage from an early change, particularly since you lease the car and it is therefore included in the cost of ownership. For the track-day, I would make sure that your oil is topped up, because you will be cornering harder and braking harder than normal, and this does cause the oil to 'shift' a lot more in the sump.
Definately change the brake fluid to the higher-temperature, such as the motul suggested or ATE Super Blue brake fluid.
No need to fit braided brake hoses, the S5 hoses (at least in the UK spec) are already braided under the plastic sleeve.
Pads... Caution, EBC yellow-stuff is track day only as when cold it doesn't perform. Red-stuff isn't as good, but you can keep it on the car on the road too. Green-stuff is not only useless, but it's not up to the job on a car over 200bhp. Personally, I'd go for Pagid... if it's good enough for Lamborghini, it's good enough for the S5!!
Have a great day and definately get some instruction... it will make you much faster than several thousand dollars-worth of modifications!
