Past incentives have ranged from meal vouchers to gift cards to...virtually nothing. I don't think this is a situation where you can tell your friend "if you take the tour you will get ____ ."
I don't think there's anything wrong with stopping by one of the kiosks and asking if they would offer any incentive for taking the tour. Depends on how bold they want to be.
Do let them know that Disney will arrange round-trip transportation to minimize the time commitment. They can pick them up right at their hotel (or any other origin), take them to SSR for the tour, and then return them to any other destination on WDW property. Reportedly they will offer better incentives to those who agree to tour during the normal slow periods of the day (which I assume is around mid-day when most folks want to be in the parks.)