What do they like? I would suggest a quick "wrap" type sandwhich with cream cheese spread on the wrap, followed by turkey, ham or smoked salmon, some greens, pickles or cucumbers, tomatoe and a tiny bit of ranch dressing. You can slice tomatoes days ahead and refrigerate them. The meat is already prepped and you can get those bags of already mixed salad.
Another good one is cottage cheese with drained, canned fruit and a few chopped nuts added.
Split bagels, spoon on some pizza sauce, spaghetti sauce or tomato sauce (add herbs or spices as you wish), sprinkle on a lot of grated mozzarella, and top with whatever they choose - pepperoni, pre-sauteed ground beef, onions, olives, mushrooms, etc.
Grill under the broiler until cheese is toasted and bubbly, but watch carefully. The time required depends on the distance of the broiler shelf from broiler elements.