We do the things you've mentioned above, yoghurt into a re-usable container, sandwich in lunch wraps, museli slice cut up as museli bars etc... it can be done and your best friend is going to be organisation. Get lunches ready the night before as much as you can and you won't be grabbing the pre-packaged stuff for convenience.

We use a thermos container for lunch most days, summer it has fresh fruit (watermelon, rockmelon) in it, winter ds quite often takes leftovers for lunch, fried rice, meatballs, pasta etc...