As far as left-overs, you'll be suprised how good stuff tastes, even when it is cold! A lot of things you usually eat hot, stir fries, meats, etc, still taste pretty good in a bento. One of the best bento I have ever had was on a shinkansen platform, and it was some kind of thinly sliced stir-fried pork.
I think the more ingredients you have in a bento, the better they are. When My host mother in Japan would make bento, they would always have leftovers, rice, tsukemono, vegetables, etc. Personally I think beef tastes good cold, if you had made sesame beef or something the night before.