I can't really help with the vegie burgers or tofu, but there are other things they would also enjoy I'm sure.

Stuffed mushrooms as mentioned.

Corn cobs in foil ... could make a special sauce or mayonnaise to drizzle on top if you didn't want them plain.

Stuffed potatoes ... steam potatoes in skins until just tender. drain and cool slightly. cut off the top of pots and scoop out the pulp and mash. cook some spinach. add grated parmesan cheese, shallots and a dash of nutmeg. mix it all up with the mashed pototoes. place each potato in foil securely wrapped. Barbecue until heated all the way through.

Could make little vegie parcels the same way. A mix of steam vegies with some butter garlic and herbs (like basil) and top with cheese.

If they are vegan leave out all the cheese and butter

Stuffed zucchini or stuffed tomatoes.

asparagus cooks nicely on the barbie.

could make fruit skewers. Pineapple cooks great on the barbie too.