I would like to create a simple "stock and flow" modeling environment.  Stock and flow models can be as simple as predator/prey population fluctuation to global warming models following the carbon cycle.  Such models are great education experiences, playing what if experiments, and the output could be charted in a table or used to drive a sonification or visualization of the model.

It seems almost trivial, stocks are accumulators (up or down) and flows are controlled by valves (multiplier), and a clock to step the model.