Nearly all prices are set in US dollars save for the public transport fares. And if you are calling a taxi, inquire about the currency you will have to pay in.
Nutmeg is the thing you should definitely buy here. And grab some other spices as well. Take as much as you can. They are very cheap here and of unbeatable quality.
Getting from the airport to the capital on foot is quite possible. Besides, you could meet a taxi driver on your way who would talk you into giving a ride quite cheap. But never take a taxi from the airport.
Grenada sidewalks seem not to be designed for pedestrians. They are too narrow, and almost missing, that’s why long walking tours are hardly probable. Think about renting a vehicle.