This one I've been doing for a while and HB's always love it. The best part is that it takes them a while to figure it out... so here it goes.
Tell them you want to play a fun game with them. Take 3 pieces of silverware (usually a fork knife and spoon) and randomly arrange them in the middle of the table. Tell them that the arrangement is supposed to represent a number between 1 and 5.
After you lay the silverware out pick a number between 1 and 5 that you want it to represent. (It's safe to completely make it up.) Make the number on your fingers and place it on the side of the table next to you. At the beginning of the game she won't notice you doing this.
She will probably guess wrong right off the bat. Tell her she is wrong. Rearrange the silverware again into another random sequence. Pick another number and make it with your fingers and place it on the table next to you. She will probably guess wrong again. If for whatever reason she does guess right and doesn't notice the hand gesture simply change it quickly so she doesn't notice. If she notices the hand gesture and guesses right then applaud her.
So now she's been wrong twice in a row and will become even more determined to try to figure it out. Continue the game as usual, but progressively start slamming the number down more emphatically onto the table the further into the game you get. Eventually she will notice you slamming the correct number onto the table. Then she'll get a smile on her face and become excited. At this point make fun of her for being unaware.
Have fun with it guys and let me know how it goes.
-Slayer