As a U.S. citizen you can waltz through immigration (my last entry took about 7 seconds; plane got to the non-Schengen gate from Dublin at 8:13pm, walked super fast, no queue at immigration, and still made my 8:45pm connection to Hamburg - - even though KLM had already rebooked me for the next morning!) then stay for 90 days.

Technically it is 90 days within any 180, so you can stay 90, then go away 91, then come back. (Or you can do what I do and stay 40, go away for 42, then come back for 40 etc. into eternity.)