Make a big pot of mashed potato to accompany dinner one night, and use the leftovers the following night to put over meat in the oven as a different dinner, as in:
cottage pie,
shepherd's pie,
over a chicken and mushrooms in white creamy sauce pie, or
over smoked white fish (haddock/cod etc) already poached in milk, use the poaching milk to make a white sauce for the pie, flake the fish and cut broccoli into small florets in the flat dish, cover with the white sauce, top with mash, scatter a handful of breadcrumbs and then a scattering of grated cheese over the top for crunchiness.
Homemade chips.
Homemade wedges - season with a pack like Schwatrz, with any fajita type seasoning you have, or just (garlic granules, dried rosemary, lemon pepper and salt), and a good glug of olive oil.
Use for Spanish omelette for lunch.
Use leftover mash to make crab cakes (tinned or fresh crab meat) with some spring onion and tinned corn (we've made tuna cakes the same way), mould into patties, give them the traditional "flour, egg, breadcrumbs" treatment and then fry in a pan. (Lovely served with salad and sweet chilli sauce).
Potato salad - I peel the potatoes before boiling, and then throw in my sauce when they are drained and still hot to absorb into the spuds, and also have spring onion chopped into the bowl. Sauce is a couple of tablespoons of Hellman's mayonnaise, tablespoon of crème fraiche or natural yoghurt, and good pinch each of salt and pepper.
Gratin potatoes with cheese and meat scattered into the layers as a single baking dish dinner (we do one with stilton and leftover ham as a Christmas leftovers treat). Not just as a side dish.
Bombay potatoes with a curry night.
Baked potatoes are really handy if you have the oven on anyway - DH has been doing a few some evenings to then reheat in the microwave to have as lunch the next 2 days with something like baked beans, leftover spag bol sauce, leftover chilli con carne sauce, tuna, or just grated cheese, on the top.
I often use leftover boiled potatoes as homefries - peel the skin (optional), slice into thick slices, fry on a medium heat pan until both sides golden - nice with a fried egg or rasher and sausages.
I also do "cheats' roasts" sometimes midweek (in normal working outside house rush) - peel and dice potatoes the night before and pop into a Ziploc bag with garlic granules, lemon pepper, dried rosemary, good glug olive oil; close bag and give a good squish about so all potatoes have some oil on them and seasoning is well distributed. Have oven set on timer to heat before you get home. (Also have your chicken joints - whatever you have of oyster thighs, drumsticks, legs, part-boned breasts; or lamb chops/shanks; well seasoned in another bag; and possibly a 3rd bag with veggies for roasting prepped and seasoned). When you get home, throw them all onto oven trays and into the oven for 30-40 minutes, toss the potatoes once mid-way to get lots of crispy bits, veg usually needs less cooking time so add that at the same time as tossing spuds, and you have time to deal with emptying work bags, (dealing with homework), checking postbox, having a shower/changing, pre dinner drink etc as needed before eating rather than slaving over the stove.