Marriott Courtyard Santo Domingo?

Metro Courtyard is in Ave. Maximo Gomez....it's not in a walking distance from Zona Colonial, you would have to take a taxi or a carrito publico going parque at the Entrance door in Ave. Gomez.
The Metro bus terminal is far from the hotel but if you take a cab you could get there in 15 minutes (depending at what time you leave the hotel due to traffic of course). Cab would cost from 130 to 150 pesos so make sure the driver doesn't charge you too much just because you're a tourist.
The area around the hotel is safe if you go North, southwards....not very much. Ave. Maximo Gomez is a very important artery in the city. There's always a lot of activity, especially close to Ave. Bolivar. However, it is close to the US Consulate, which is located to the north side, that area is pretty secure and calm.
I don't know how much a cab would cost from hotel to Las Americas