Chicken fajitas
Mac'n'cheese with bacon and veggies in it
Roasted chicken joints (thighs or drumsticks - not part-boned breasts as they need 45 cooking time) and diced roasted potatoes, frozen veg (steamed or boiled) or wilted spinach
Diced chicken or whole prawns fried, (toss in veggies if you have the energy to chop), toss in jar sauce, serve with rice or pasta as suits sauce
Grilled chop, boiled potatoes, steamed veg
By "not having been gome to prep beforehand", do you mean you
a) all get in together from work/school having left that morning, or
b) arriving into somewhere you don't normally live and need to get food made in a hurry for people with you or that you're visiting?
Cos if it's (a), you can make sauces the night before after dinner, and leave ready in a pot to reheat while cooking rice/pasta/potatoes.
Clean and chop veggies and potatoes, either storing in a pot/tub soaking in water, or in an airtight plastic tub/ziploc bag, depending on the veg (spuds need soaking, onions/garlic/peppers/mushrooms/courgettes/parsnips need airtight, carrots/sprouts etc prefer water but manage either, while brocolli/cauliflower prefer airtight but manage either etc).
Take sauces out of freezer to defrost, as per 1st suggestion
Organise a traybake option in the morning (or prep night before and just toss into baking tray in morning) and set up the oven on the timer to be cooked and just turning off a few minutes before you need it - this also works for stews that you throw in a casserole in the oven and ignore, don't brown the meat, just toss with veggies and either stock/seasonings or a packet of sauce in casserole dish, give a good stir, put on lid, ignore except for setting oven to turn itself on and off again and head out for your day.
Also, good either for oven on timer, or throw in as soon as you get home - cottage pie, shepherd's pie, lasanga, chicken and mushroom (or leeks) in creamy sauce pie (with potato or pastry top), smoked fish and brocolli pie (potato top) etc. Useful to do a very large pot of mash 1 night for dinner, and when cleaning up make the "sauce" part of suitable dinners and chuck together in an ovenproof dish for the following day. Or make cauliflower cheese as a veg 1 night, but a large pot of sauce to easily layer up a lasagna (especially if you have made a large batch of bolognaise sauce another night...) for the following day. And almost all of those also freeze well.
If (b), then it's more about bringing things with you that are easy to cook.