Perhaps the best new ideas are born when two old ideas intersect. When you're trying to think of an idea for a science fair project, why not think about doing a mash-up of two sciences, or of two ways of looking at experimental science. You'll want to develop your own idea, but here are a few concepts to get you started.
Psychology Meets Robotics
Normal psychology is interested in how ordinary people react to their environment. Why not do a study of the features that make a computer likable. Is it something in the features, like having two eyes rather than three? Is it the robot's shape, being shaped like a mammal and not an octopus or a snake? Or is it the way it sounds, or feels, or even smells? You'd only need a touch of robotics to do this project. Some people find the way some sports cars headlights pop up endearing and human-like, so if you could find a way to make a robot open its eyes, you could compare people's reactions to, for example, an unblinking rectangle, an awakening robot, and a robot going to sleep. Or perhaps you are interested in what features would make a robot seem more trustworthy, or would make it startle people.
Population Studies Meets Film
Population studies are done of bacteria as well as of people. If you have the equipment, you can produce a film that compares the growth of bacteria to the growth of a city. You could use a series of maps to show the growth of a city, and intercut it with time-lapse photography for the growth of bacteria. Maps of the Dallas-Fort Worth area could be compared to what happens when two colonies intersect. So could a similar selection of old maps from your own area. St. Louis or Manhattan could show what happens when land is limited. A librarian can certainly help you find old maps. Filming bacteria colonies might be harder to do, but scientists often do it.
Mturk Meets Curiosity
If you can think of interesting questions to use in a survey or quiz, you can put it on Mechanical Turk, mturk.com, for less than the cost of materials for a constructed project. People will answer questions for a penny or two, if they are quick questions. Or you could offer a quiz, or request art work and put together conclusions about people's responses. You could try to find out if people will work twice as hard for two cents as for one.
History Meets Geography
Do great civilizations destroy their environment, or do they enrich it? Many ruins stand around the world where mighty nations once were. Did they deplete their environments? Is the former territory a wasteland because a civilization stripped it, or because it is no longer a garden? How could you research this?
Rube Goldberg type devices are always popular at science fairs. Everyone is interested in the way things fit together. A complicated device, with some humorous aspect if possible, can be the star of a show.
Paper computers are fun too, and not that hard to do. They illustrate the operating principles of computers in a way that can be quite absorbing. That's an example of a project that costs almost nothing to do. It's made of research, patience, and paper.
If it's allowed, why not work together with someone. Your different points of view may help you to think of a great idea and carry it out brilliantly.