Need a pair of compounds that is similar in formula but different in structure
The classical example is dimethyl ester $\ce{CH3-O-CH3}$ and etanol $\ce{CH3-CH2-OH}$ pair. They have exactly same brutto-formula, but different structure and very different properties. The first is a gas, the second is a liquid easily reacting with sodium metal. 

Carbon is also interesting example. It has two common forms with exactly same chemical formula $\ce{C}$, but very different properties: a very hard and transparent diamond and soft black graphite.