Internet travel guide 1 travel USA Cities links Comprehensive listing of U.S. States and information on travel etc. Marie plans trips to Sweden every year. Take a look, it may be what you want to do. Buy a City card. It is reasonable and gives you free passage on the bus, free entrance to many museums, the royal palaces, other free rides etc.
Car rentals are somewhat expensive.
You CAN use your current drivers license. You do not need the international drivers license. Parking places are hard to find in the city, and it is expensive to park. Some of the hotels make you pay for parking in their lot or garage. You might want to check.