I think you would be most likely to get a Playbill back; I know that some people have gotten them without donations, signed by the cast. You could even try writing two letters: one to the stage manager requesting a Playbill signed by the cast, and one directly to Bryan Cranston... explain that your boyfriend is a huge fan, you guys saw the show, and you were wondering if he would sign the playbill? Maybe he would personalize it to your boyfriend, which would be a cool bday gift
((btw, I've never tried sending a window card, which is why I didn't answer that part of the question!))