A main reason many people like to, or want to, make bento lunches is for more variety, to save money, and to have some fun too. In my mind these aspects are quite interconnected.
There are three sources for filling your bento box. One is food that you make specifically for it, usually in the morning or perhaps the night before. The second is leftovers from other meals. The third is with stock or staple items (aka johbisai). The key to keeping a good variety in your bento meals is to use all three sources in in a smart way.