The trickiest part to all of this is the containers, esp if you're trying to steer away from plastic bags, etc for the sake of the environment.

I learnt fairly early on that DD was a bit prone to leaving my tupperware items around, so get used to rummaging through the lost property box regularly (in the classroom and in the office) or stocking up on new ones every so often.

You can freeze your own muffins/slices/ready made sandwiches, and they come out fresh & lovely.
You can also freeze yoghurt in summer, keeps the lunchbox cold or they can eat it still frozen for a change
DD1 quite likes to take cold left over pasta from the night before.
A mixture of a few different things (ie crackers in together with dried fruit) provides variety
We send hot milo or soup when it's really cold in winter.