Nearly everything that makes a home look considered is a matter of editing and placement rather than spending. These are the changes worth making first.
Change the bulbs
Warm, dimmable, and the same temperature throughout. It costs very little and it changes every room in the house at once.
Raise the curtain pole
Closer to the ceiling, wider than the window, fabric to the floor. Same curtains, taller room.
Decant the everyday things
Hand soap, washing-up liquid, coffee. Removing brand packaging from the surfaces you look at daily is the cheapest visual upgrade there is.
Group in odd numbers, vary the height
Three objects of different heights on a tray will look arranged. Four matching objects in a row will look stocked.
Buy branches instead of flowers
Eucalyptus, olive or something seasonal from the garden lasts weeks rather than days, and the scale of it does more for a room than a small bunch ever will.
Fix the one thing you have stopped seeing
The scuffed skirting, the door that does not close, the picture hanging slightly crooked. Visitors register these instantly, and you stopped noticing them years ago.