If you want to order a special meal, you need to give the airline some
advance notice (how much depends on the airline).
If you are delayed, bumped or switched to another airplane,
you may not get the meal you requested so it's always a good idea to
pack up a bag of food that will keep your stomach from complaining.
I would also recommend the fresh fruit plate served by many
airlines. Depending on the airline, this makes a refreshing snack-type
meal. A lot of the time, it does come with cottage cheese on the plate
with the fruit so people who don't want it might be better off with
the veggie meal. I've gotten wonderful melon/pineapple plates and
yucky apple and unripe fruit dishes. Depends on the airline.
Sometimes there are vegan or lowfat meals offered also but the lowfat
might not be veggie.
I don't know if there is a list of available entrees anywhere but
airline web pages might have something.