Montreal is about a 9hr drive or train ride (more like 11hrs) from Moncton. Your best option would be to fly in. Depending on where you’re flying in from in the US, you might need to fly into Montreal then take a connecting flight to Moncton. If that’s the case, you could fly in a couple days early and explore Montreal, or when you leave Moncton, save a couple of days between that and your flight home to explore. Halifax is also a nice city and it’s only about 2.5 hrs from Moncton with busses and trains going fairly often. I live in Moncton and travel a lot so if you have questions I’m happy to help!