The best way to spend your money would be to improve your nation stats (literacy, crime, disease) and save for new cities. Lower crime and disease rate while increasing literacy gives bonuses to population happiness and immigration, increasing nation growth. Buying a new town will add 10k new people, upgrading it to city will boost it to 50k and upgrading a metro will boost it to 150k people - thats a lot of new citizens, which means more money and theoretically a higher growth rate (more babies!).
Buying land is also good to make sure you have a good population density. This also factors into immigration and the death rate (lower death rate = more growth of course).