I agree with Granmarderby10. I was just about to say, clean windows can make a huge difference.
Could you tackle one room at a time. Assuming you are in the UK, Asda, B&M, The Range, all have cheap items such as bedding, lamps etc.
Depending on what is on your walls at the moment, would you be up to giving them a coat of paint. You can watch YouTube videos for advice on how best to do this.
If you decide to concentrate on the living room, a few new cushions and a throw can change the look of a room. My living room has neutral coloured walls (Dulux Timeless) but I change my cushions colours quite regularly. I have made my own cushions and bought some. I use the same cushion insides and just change the covers.
I also agree with kittylester - you can get a bunch of daffodils for £1 at Asda at the moment. I have picked up loads of lovely vases from charity shops for next to nothing. And yes, open the windows and let some fresh air in.