my kids realy like finger type foods at that age so some of the more popular items were-
veggies and ranch dressing (get the tiny reusable containers that the lids snap solidly on)
a peice of lunch meat (usualy turkey or ham) with cream cheese inside rolled up
string cheese (can also be rolled up in lunch meat)
bean burritos (heat the beans and fill the tortilla with it and the cheese-wrap it in foil to keep somewhat warm-seemed gross to me but the kids liked it)
"nachos"-a bag of tortilla chips and a thermos with warm refried beans sprinkled with cheese
for yogurt i stick with those "tube type" containers-they tend to eat all of it
ants on a log (celery with peanut butter and raisins-sometimes we used 'craisins' instead-"fireants on a log"

)
chicken nuggets (again it seemed gross to me to eat them cold-but they wolfed them down, so i cooked them in the morning and wrapped them in foil) with a container of ranch (i started getting smart and setting aside the containers that came with their happy meals-they used the bottled stuff at home and i sent the fast food containers to school

).
ds is a pastaholic-so i keep cans of chef boyardee on hand-nuke it and stick it in his thermos (the wide mouth type is best-just make sure you get a lunch box thats big enuf to hold it).
tortilla 'sandwiches'-the kids fill the tortilla with the ingredients (usualy veggies, maybe chopped up left over chicken and cheese) roll them up and off they go.
our superwalmart sells small containers (maybe 3 to a package) of apple slices with dipping sauces (carmel sauce is very popular but it's a once in a very rare treat, but they also have them with peanut butter)-they are actualy reasonably priced. they also have similar packages with baby carrotts and ranch dressing. i also get the small boxes of raisins, actual "fruit juice" fruit treats, and for cookies-i get the ones that are marketed to dieters-nabisco (i think) has these thin versions of oreos and other types that only have a very small amount in each bag-they have the same thing with wheat thins and some of the other crackers.