I love tacos!! I make them weekly (really, chicken fajitas).  I seldom have lettuce in my house, it just doesn't last long enough (can't stand iceberg lettuce, ugh).  On one of the cooking shows about food trucks, the taco guys were using finely sliced cabbage in lieu of lettuce. I tried it and it's really good! Cabbage lasts 10x longer than leaf lettuce and it adds a nice crunch.