I think for me, it's got to be the sugar free pecan ice cream*, at most of the permanent ice cream parlours (i.e. not the little carts). I'm not sure if they have a sugar-free cone or not, but you can play it safe and have it in a cup instead. Just don't make the mistake we did, and try to eat it any where other than inside or just outside the parlour - they were half melted by the time my partner (a very fast walker) had carried them from Main Street to Liberty Square

!
There are also a lot of good sugar-free treats at the confectionery stores - some of them even have a whole sugar-free section on the counters! If you're in a restaurant and can't see anything suitable on the menu, don't be afraid to ask. I'm a vegetarian, and often there is no veggie option listed on the menu, but they have one available 'out back'; it may well be the same for sugar-free stuff.
If you're planning any special meals at a TS restaurant, you should email Brenda Bennett (
Brenda.Bennett@disney.com ) in advance. She'll be able to put you in touch with the chef for that restaurant, and they can talk you through all the options, and maybe get something special in, just for your DW. I've heard nothing but positive reports from everyone who's done this, so I hope you find something wonderful!
*I'm not diabetic, so I'm not an expert on carb counts, I just find that sugar can aggravate my condition. Please do all the normal checks that you would usually do; don't just take my word for it!