I use a hot plate (platta) for Shabbos and Yom Tov. On a typical Friday night wheat the food ups. The oven and then place it on the hot plate. We actually use an inverted cookie sheet so it shouldn't burn (they get incredibly hot). Shabbos morning we put some Kugel, deli roll, schnitzel etc. on a few hours before we are ready and it's always piping hot.
This year we are going to trait with Dairy. Double wrapped the hot plate, and also the food. Hopefully it will warm up well. We also left a small burner on with a blech which we can use to heat soup, vodka sauce, and make pasta.
We never leave the oven on especially in the summer. It makes the whole house unbearably hot.