fried tofu with the spices of your choice (i often do chicken seasoning. tastes like fried chicken if you fry it golden. so trashy, so tasty.)
pesto, sundried tomatoes and vegetables/lettuce/whatever. roasted/grilled veggies are great for this (make some for dinner, and put the leftovers in a box in the fridge for sandwiches).
tahini and blueberry jam (SO GOOD, especially if you toast the bread)
bananas, margarine (i'd prefer butter, but for vegan use non-dairy margarine) and a sprinkle of salt. or peanut butter, if you want to do it elvis style.
mustard, black pepper and mushrooms.
deconstructed guacamole sandwich: slices of avocado, tomato, and onion topped with cilantro, salt and lime. for extra crunch and reduced nutritional value, add tortilla chips.
although i don't like most "specifically vegan" imitation-of-something-or-other-products,
tartex is actually very good. if you can get something like that where you are, try it. seriously tasty on toasted bread with a few slices of cucumber,