While not quite no-carb, white potatoes are high glycemic index without resorting to sugary snacks. A bag of chips at the convenience store is a reasonable alternative to donuts, etc., if you're tired of sweet stuff.
Two or three years ago GCN did a long distance test (100 miles, I think) with Hank eating normally while riding, with Conor fasting while riding with a glucose monitor. Toward the end of the ride Conor finally tested a few snacks and they discovered a packet of chips (or crisps?) restored his blood sugar as quickly as a typical sugary snack or sports-specific snack (gel, etc).
If memory serves, sweet potatoes/yams have a lower glycemic index than white potatoes, despite being "sweeter."