Basic First Aid kit, including over the counter medicines for stuff like colds and flu, if you get ill or cut yourself or burn yourself you're on your own and these aren't the times you want to be going shopping because you've got nothing in to help.

Emergency food, same reason as above, if you've got the flu and just want to mope in bed for a week there's nobody there to look after you. Just simple stuff to survive on, but stuff you don't want to be stuck without. I keep tins of baked beans, tins of soup, tea-bags, a carton of long life milk, and some bottled drinks in a separate cupboard from most of my food and a loaf of bread in the freezer. It's hardly fine dining but it means if I am stuck in at any time I know I've got something quick, simple and warm to put inside me to tide me over.

Torch, with batteries, powercuts happen, you need something you know where it is and that it works.

Plunger. Reddit always says plunger so I'm not going to one who doesn't include it.

Not something you need buy but definitely something you need to do, clean and tidy up after yourself. Particularly if living alone things like washing-up when there's one plate, one knife, one fork and a cup seem like such a pointless endeavor... but you put it off and it keeps getting put off until it's a big job. Likewise hoovering, dusting, etc. draw up a room by room rota and stick to it, so easy to think the whole place is clean so nothing needs doing... but when it does need doing everything needs doing, far easier to hoover one day even if "it'll be alright for another week" clean bathroom the next, dust the next, you basically lose like 20 minutes a day, rather than suddenly finding you've got a task that'll wipe out a weekend on your hands.