Here are a few games you could play:

"Guess What I'm Wearing Under My Pants?"
"You Tell Me Your Secret and I'll Tell You Mine."
"Do You Know Whai I Want for My Birthday?"

Or you could crank up your courage and just tell her the truth. It won't be easy but it is the right thing to do. Tell her before summer is over, she won't be as suspicious of your heavy perspiration as you are working up the nerve.