Do some test flights and measure the distance your planes fly.  

Try different designs.

Measure how far your vehicle will roll.  Build a ramp and now measure how far your vehicle can roll.

How many pennies will your box hold?  What else will it hold?

How can you redesign it to hold more?

How many pennies will your bridge hold?  What else will it hold?  How can you redesign it to hold more?

Did your mini-figure land safely? What else can your basket hold?

How does your instrument make a sound?  Compose a song.

Conduct some launch tests.  Measure the distance your projected object sailed.  How can you redesign your catapult to work better?

Cut up magazines to make squares for origami paper.

What else can you make?

How far can your rocket fly?  

How can you redesign it so that it goes farther?

How does your toy work?  

Explain it to someone else and have them play.