Hi Ana,
Welcome back! It is mostly a great city. Anyway, I highly suggest you buy a Smart Trip card at the closest metro station when you arrive. Not only can you use the metro, metrobus, regional buses and finally the DC Circulator but you get some discounts when transferring from one to the other. It is very convenient. I am not sure where you would buy the all-day Circulator pass.
The discount would be 50 cents from Metrobus to Metro. Not sure if the DC Circulator is included for discount.
Only way to pay for the metro is using the Smart trip card. There are machines at every metro station where you can get smart trip cards.
Have a great trip!