Fresh flowers and succulents from the garden is my go-to cheap decorating solution.
I also like kids art! As well as making inexpensive frames you can always buy canvas frames and get kids to paint directly on them.
Here's some other suggestions - https://www.domain.com.au/advice/ingenious-designer-decorating-secrets-that-wont-break-the-bank-20170123-gtwyg1/ - the seagrass rug suggestion for large areas is a good one.
What are your solutions?