Make a physics problem solver. Sometimes called an equation solver.

Let's say you have a simple kinematics question:
Bertha jogs at an average 7 miles/hour, and her typical path is 2.5 miles long. How long will it take her.

You would let the user enter three items:
1. 7 miles/hour
2. 2.5 miles
3. ? hours
Your program should figure out the formula and fill the question mark.

A more advanced version would convert units and standardize them. So the user can enter:
1. 7 miles/hour
2. 17 minutes
3. ? feet

Once you get the hang of it, you can go from simple kinematics to dynamics, force, work, gravitation, magnetic flux, thermodynamics, quantum mechanics or to relativity.

Back in the dark ages when I took physics, I wrote one of these in a then brand-new language called C.