If it's going to be a busy time then you could do worse than staying a night at an on-site hotel. When you do that you get 2 days (arrival day and departure day) of unlimited Express passes thrown in, and early entry to Harry Potter. Plus the on-site hotels are ace. I stayed in the Hard Rock hotel for one night and it was a superb hotel. Really enjoyed it.
It was fairly pricey (around £160 for the night I think), but when you work out the cost of normal Express passes (which are only valid for ONE go on each ride, compared to the ones you get when you stay which you can use as many times as you like) it's actually about the same as the hotel stay anyway (for our family of four anyway).
Note that Rip Ride Rockit, Forbidden Journey and Pternadon Flyers can't be used with Express Passes