Link, that's a great idea with the fan thread and I think sneaking in tickles on your Aunt has potential. Especially in the kitchen where you could ask her to get you a glass from the cupboard. As she reaches up to grab the glass and her back is turned...well you get the idea.
Another strategy would be to jokingly tell your Aunt, with an evil grin, that you intend to tickle her again when she least expects it. Again, gauge her reaction. If she welcomes the idea (though she'll act like she doesn't) it will be expected of you to try to tickle her but you still get the element of surprise.
If she's REALLY into the idea I'm thinking you might notice her, in a subtle way, make it easy for you to get her. Examples: Raising her arms to stretch while within reach of your fingers, taking her shoes off while sitting next to you on the couch and crossing her legs so her raised foot is closest to you. She'll know what's coming but not exactly when. You'll still get the surprise. Win-Win.