DS1 is in reception now.
You've got lots of foodie ideas here so I won't go there with that, but my feedback would be:
Make sure your child can manage everything in their lunchbox by themselves. If they have to wait to ask someone to peel their orange/open their yoghurt etc etc, they'll never get done in time.
Don't put too much in. Ours have a half hour window to eat, which sounds plenty but by the time they sit down, say grace, have a chat, faff around a bit, before they know it time is up.
Put sandwiches in little freezer bags rather than clingfilm, and secure shut with a sticker. Nice and easy to get into, then train your child to put all the leftovers in the bag. Like someone else said, school will send back everything, eaten or not, so unless you want crusts / yoghurt lids / banana skins exploding all over the lunch box, train them to put it in the bag. When you unpack the lunchbox at the end of the day, lift out bag and place in bin - voila!
Find out what they get given at snack time, so you don't repeat it at lunch. At our school they get offered a variety of fruit, but DS always chooses apple, so he doesn't get this for lunch.
HTH 