I like to make the honey-scallion mixture ahead of time and then just grill the vegetables when ready to eat. The "bite" from the scallions really brings out the flavor of the honey. The most important part of a skewer to get right is having everything cooked at the same time. To that end, I pick three vegetables that have a similar cooking time. Undercooked onions can ruin a skewer, for example, so I cook those a little, first, to better match the shorter cooking time of the other vegetables.