Looking at the menu, it seems that the fruit skewers are the best but since you don't want them I'd say the next best thing would be the strawberry or raspberry 'Sundae Best' at 248 calories. That would equate to about 12.5 syns.
Next to that is the Knickerbocker Glory Sundae at 350 calories. If the raspberries, strawberries and pineapple in it are fresh, unprocessed fruit then that can be counted as free. You could estimate the calories in the fruit (about 10 for a couple of strawberries, 10 for a few raspberries and 40 for a slice of pineapple) and deduct that from the total to give you about 300 calories to syn. That would give you about 15 syns for the dessert.
Other than that, the rest of the desserts look painfully high in syns.