See/Say/So
.
The "victim" is restrained. The top has a note pad and pen. The top explains to the bottom that he/she is going to write one of three words on the pad each time--either "see", "say" or "so". The bottom has to correctly guess which word was written six times in order to be released (or whatever reward is decided). Simple?
The penalty for the first wrong guess is two minutes of tickling. Each successive wrong guess adds another minute to that guess's penalty. If you've had a little experience with probability and statistics you can calculate the odds of how many guesses it will take your play partner to come up with six correct guesses. Suffice to say that the matrix is large enough that you will have plenty of tickling time, and depending on their endurance you may have to set a ceiling on the number of tickle-minutes (or maybe not...).
.