Our DD4 is a huge pressed penny fan too - she’s far more interested in these than most of the things in the gift shops!
We set expectations before leaving, e.g. she can have one princess dress and one toy. This has always worked well. If she sees a toy she likes I’ll make sure she understands that she won’t be able to get another - is this really the one toy she wants? A lot of the time the answer is no, but when she finds something she likes she does stick with it and doesn’t ask for anything else. We do have a bubble wand and a couple of spinny light up toys which we take with us, but she’s been less interested in those the last couple of trips, to the point where the bubble wand will be staying home in September.