I have used cute scrapbooking paper in frames as artwork in the girls rooms. There are several designs with coordinating colours/motifs. Big W & kmart often have cheap frames too.
Paint is one of the cheapest and yet something you can get the most value out of.
In my unit I've painted all the walls, repainted the old medicine cabinet inside and out with spray paint (pink in the inside!), painted furniture, pictures.
But if you can't afford paint, you can often makeover things and rooms by cleaning them. Clean as much as you can by putting things in a washing machine or dishwasher if possible.
Plants also add to a room, whether they're real or fake.
Hooking things around curtain rails.
Recycling containers to be used for another purpose.
Bookmarks