It's a while since I've done it but it's fantastic - not just for working parents but also SAHMs if you have a stash and are going to be at the park at lunchtime. Much easier to just take one out in the morning and pack it up in a picnic.

In the past, I've done:

Tuna mix - so tuna mixed with mayo, avocado, corn.
Turkey, brie (or any cheese), cranberry, grated carrot and a spread of mayo
Roast lamb and pumpkin dip