One thing my son is liking now are lettuce wraps with chicken in them. I just take a leaf of lettuce and wrap it around some shredded carrots and slices of chicken. He doesn't like mayo, but you could easily add that or some dressing. I got a little tupperware container that has a cold-pak built into the lid. You just put the lid in the freezer overnight and it keeps it cold the next day. I know a lot of kids like 'homemade' luchables too- some crackers, some cold cuts and pieces of cheese to put together themselves. My son also likes yogurt tubes, which I put in the freezer so they are thawed but cold by lunch time. We've been mixing these options up with basic PB&J and goldfish crackers, pretzel sticks, fruit cups, applesauce cups, grapes, strawberries, veggies and dip, apples, etc. He seems to like all of it and eats pretty much his entire lunch, so I think its working. Also, if you have a Trader Joes near you, they make these really great super healthy little cookies called Righteous Rounds that come in an orange bag. They are packed with fiber and other good things, little chocolate chips, low in fat, etc. and SMALL. I give my son a couple of those too as a dessert and he loves them- doesn't even know they are healthy!!
I noticed a lot of other people said to look at bento sites. We got a bunch of little bento box containers and my son LOVES them. He is the envy of the other kids with his interesting little lunch containers and the bento sites do have a lot of great ideas. We are pretty much totally weaned off of those little disposable plastic bags, so my son is now part of the 'waste-free lunch' that they are encouraging at his school. Bento rules!