Interesting question, you see the story a hundred times in the opposite direction (male-to-female), but this is another perspective
Have you asked her what she finds so exciting about being a boy? I'd like to hear her intake as well.

Apart from how she imagines it, you could try having her dress up as a boy, wrapping something around her cheast to hide her breasts.
Stuffing a penis-form in her boxers or use a strapon. Not letting her sit on the toilet, but pee standing up. Let her try and give herself a blowjob, we all tried it once
Depending how far you wish to take it... cut the hair, don't shave legs or armpits.
Fix things in the house, assemble an ikea cupboard, do it all in a jeans and tank top, don't shower for a week, watch porn, walk around in nothing but boxers and white socks, leave the dishes until there's something green growing on them, what else do we guys do?

How do you see it yourself? What do you want to get out of it?