I recommend dressing up as one of Patti's earlier roles. Mrs. Lovett is a fun one, as is Reno Sweeney. It will show her that you really respect her as a performer and affirm the fact that you are a super fan.
Oooh the best idea is to dress up as Norma Desmond, that'll bring up some great memories, or maybe you can just dress up as Andrew Lloyd Webber. She loves him.
/s
Anything regarding shows stated by this account is an attempt to convey opinion and not fact.