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.

A set of model 'cells' containing 'DNA strands' that can be folded into different arrangements to illustrate the importance of DNA folding in gene activation and so in determining what cell types are made.
Build your own machine with gears, pulleys, cams, and other mechanical devices.
Using the cello
Interrupt a laser beam and you can produce clicks, pull a piece of mesh or patterned fabric across it, and you can make notes, play tunes, or even produce speech.
A knee to mount keyhole surgery training games inside.
A table with a big lazy susan in the middle to take materials to make things with.
The Cartesian diver is a classic demo, which involves both pressure and buoyancy.
A beautiful phases of matter analogy that can show what is happening to the particles during phase changes.
Ball or marble runs are fun, but when you can reposition all the parts, and the balls are blown up to the top again, they are brilliant!
If you put a current through a wire near a magnet, it jumps sideways. The basis for all electromagnets, and therefore motors, solenoids etc.
Can you build a bridge over a river using blocks which is longer than the block itself? A simple exhibit that involves some serious thinking.