Contrary to what most people believe - the majority of AA FAs are pleasant and friendly people. What I would do quite honestly is to buy a bottle of Champagne - there are five of you - and have a word with the Purser to ask if it can be put on ice until dessert. Airlines do run out of stuff and that gesture alone will show her not only that you love her but that you try to spoil her a little. I would not go the cake route as that is quite complicated. All that said I do not know if Boston Logan has shops where you could do this - in Europe it would be much easier.