Vegemite and cheese- awful. Vegemite on toast- gods own.
A simple sanga of much virtue is the humble cucumber sandwich, and in Thailand we have pretty good cucumber. Thin white bread- very fresh, real butter, freshly sliced cucumber, salt and pepper to taste. surprisingly good, edging on healthy, and won't break the budget unless you bought a knighthood to have one.
Water and mustard cress sandwiches are nice too, once again the ingredients must be right, but I don't really know how to make them.