We took the advice of a tip I found here before our last trip.
Take those little M&M plastic tubes and fill them up with 1 penny and 2 quarters--that's how much you need for each press. My kids were 7 and 9 at the time, and they each got one tube filled up and when they were out of money they were done. We were there for 6 days--and they probably each could have had 2 tubes....they aren't that heavy to carry if you do it that way. Plus, you want to carry your pressed penny book or something so you can keep track of the pennies once they are pressed. They are oblong so of course they don't go back into the M&M container.
Its a great way to remember favorite rides at the parks....whenever my DS got off of a ride he had never done before and really liked, he started watching for the pressed penny machine.
Have fun!