wow, lots of good ideas and websites here.. thanks for asking the question though, i think alot of us moms struggle w/ healthy and fun, its not always easy!
we've found a few ideas that really work and arent too unhealthy, atleast not compared to school lunch :)
i would invest in a small container for dip: peanut butter, ranch dressing, sour cream, yogurt, etc..
instead of a sandwhich you could send pretzel sticks w/ peanut butter, or crackers and a spreadable cheese..
for veggies: carrots w/ ranch, cherry tomatoes w/ dressing, and you can get low fat/low calorie dressings, etc..any veggies are good w/ alittle dressing, you'd be surprised what they'll eat :)
fruit: apples/ peanut butter, apples/ yogurt, etc..
also spaghettios for a time was selling a lided bowl that you heat up w/ hot water in the morning, put in your warm spaghettios off the stove and put the lid on, they stay warm til lunch and offer some variety for kids that dont have access to the microwave..
good luck!