It is useful to write the equations out for your AD-AS curves, so you can easily see what the effect on the curve is (these would depend on the model you are using).
Also, you can't really start your analysis from "output decreases" or "price decreases" as they are endogenous veriable here so you would need to specify why it decreases.
GDP is usually intended as a measure of output and the interest rate is often also an endogenous variable (for example in the IS-LM-AS model).
Examples of exogenous variables may be: government expenditure, money supply, taxes etc.
With these you can do comparative statics (ie.e asking if this changes then what happens). Which varibales are endogenous and which exogenous changes from model to model, for example in the classical model output is usually considered exogenous so the analysis becomes easier.