My LO didn't get into sandwiches until more recently, too awkward to handle, but would do an open/deconstructed sandwich - eg bread with hummus, cucumber sticks, cherry tomatoes. Big fan of cold sweetcorn here too. Fruit and cheese or yoghurt? Things like savoury muffins, meatballs, veg pancakes, can be prepared in advance and frozen, then just pull out a couple to thaw in the morning.
I think our lunch at that age was most often dinner leftovers though, reheated in microwave if necessary.