A really good clean - no need to spend money but lots of time to dust everywhere (tops of books, shelves, blinds, picture frames), wash all windows and any glass/ornaments, brush ceilings and corners in case of cobwebs, hoover everywhere including pulling out all furniture, polish wood and things like door handles, make sure light switches/plugs are clean (grime can build up over years) etc.
If you can, get rid of anything you would get rid of before moving - reduce your book collection, dump broken items, bring old clothes to charity shop, etc.
Check all lights are working.
Once you think you have done the cleaning and prepping, walk out the front door with a notebook and pen, close the door, and walk back in as if you are a buyer. What do you see that works or doesn't, is there anything broken or out of place, what looks cluttered (and can you do anything about that), etc. List everything that needs attention, and also note anything that you think you would like to focus on in a positive way. If possible, try to show it as a clean, in good order and liveable space.
Even if you normally like vibrant or patterned bedding/cushions/towels etc, do you have any more neutral and plain ones that you can use for photos/viewings? Get those out and keep them clean.
Small spends may be things like a tray of pansies or autumn bedding colour if they help the garden - make sure it's well weeded and the grass is cut.
Or a pot of magnolia paint for any rooms that are tired and need brightening.
Or a cheap frame with a poster instead of the large canvas family photo.
Or a rug if there are any problem spots on flooring.
But don't go out to spend loads - you can probably borrow things from friends/family, look in charity shops/freecycle for cheap things, and only spend on things that would really make a difference. Clean and bright and liveable, that someone else can imagine their stuff in, is the best thing and that can be mostly done for free.