It's the simple things that make the most difference, imo.
Clean, clean and clean again - skirtings, light switches, door paintwork especially near the handles, loo seats down, clean inside radiators (often have years of grey grot lurking inside the grill), yellowed pendant lights - use Cif or replace with new.
No mould anywhere, ie bathroom grout / silicone round the bath / shower cubicle and conservatories (the silicone around the windows etc.)
Make the beds look immaculate and beautifully dressed, as if you were trying to pass a hotel inspector's exam (no rumpled duvets, pillows plumped etc).
Spot clean carpets if any obvious stains, or cover with a new rug if you can find one to fit the space. A new rug can transform a space and there are some lovely inexpensive ones online.
Kitchen worktops free of clutter and everything immaculately clean including the hob and oven - get a professional oven cleaner if needed (helps it smell fresh instead of lingering grease smell).
And tidy inside cupboards too - make them look organised, especially under the sink (I always look there, checking pipework for evidence of leaks etc).
Tidy and organise cupboard under stairs / coat and shoe storage cupboards, adding a movement sensor light if needed - it suggests there's plenty of storage space if everything looks orderly.
Get your painter / decorator to fill in any gaps with caulk - that makes a huge difference to the overall finish.