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We have built over 100 different science museum exhibits, mostly for the Cambridge Science Centre. These have included electrical generators, ball runs, stick insect cages, logic blocks and dissected household items.
We think that 95% of the time a small exhibit can teach and fascinate your visitors just as much as a large exhibit. Plus they have the advantage of being much cheaper, and you can get 2-3 times as many in the same space, allowing your visitors to explore more science, and making it more likely they will find something that inspires them.
Build a paper flying machine and then launch it using a tall tube. Experiment with different shapes and see how they behave.
A telescope that you can use to look around your space, change the focus, pan and tilt, and swap the lenses.