Don't exhaust yourself the first day...pace yourself. We were at the airport at 4am, arrived at POP at 1pm, at the Magic Kingdom at 3pm, into bed at 10pm...tried to cram a whole day's worth of stuff into half a day, then we were exhausted and cranky the next day. If we had taken the first half-day to swim, relax, and soak in the Disney atmosphere at POP, we would not have been exhausted already on our first "real", full day.
Some more upbeat advice...get souvenirs! Even if they're little things, they let you bring the Disney magic home with you!