Cooking - you can get really into it!
The secrets are the right mixtures of textures, and seasoning, and cooking slowly unless it's stir-fry - so that you can rescue it before it goes wrong.
I mentioned porridge, which probably sounds miserable. But think of porridge simmering slowly with a pinch of salt, half water and milk, until the oats ooze goodness. Add frozen raspberries and blueberries, a spoon of double cream, a sprinkling of toasted flaked almonds. It's heaven! And costs pennies.
For vegetables - I rely on a little butter, or a good dressing, to turn even the most boring into a gorgeous meal. Broccoli al dente with a lemon dressing & some parsley, chopped red onion or spring onion and a little red chilli sprinkled on top - really gorgeous. Broccoli bunged on a plate with boiled potatoes? Miserable, especially if they've been boiled to sludge.
Build up a stock of essential seasonings - white wine vinegar, ginger, garlic, lemon, black pepper, chilli powder, cumin, veggie stock cubes, mustard. A reasonable olive oil will do for cooking and making dressings.
One of my go-to meals now is egg fried rice (with peas and carrot), using rice frozen after an earlier meal - seasoned again with chilli and raw red onion, perhaps a little soy sauce if you have any in. It feels like junk food but is pretty healthy if you control the salt.
A tip for adding carrot to bolognese or veggie fried rice - chop it into tiny pieces and cook in shallow boiled water in the microwave for 3-4 minutes before adding (almost cooked) to the sauce or the rice.
I'm not sure if anyone has mentioned supermarkets that specialise in South Asian food. You can get big bags of lentils and rice, and very cheap veg. They often sell containers for batch cooking too - foil containers with cardboard lids, or plastic boxes. Get a roll of stickers and a sharpie. Make the labels sound enticing, so that you have something on hand when you're tired and stressed, and desperate to go for an Uber Eats (£££££).
Another secret is the right level of variety, which depends on what you need yourself. But if you are eating cheaply and healthily most of the time, you will have a bit more money available for when you really need a steak. And the yellow-sticker food does throw up treats, so there is no need to feel deprived.