I've driven from Montreal to PEI. It's a long drive, but it's pretty easy. If you're not going to fly, I'd recommend considering renting a car.

You'll be in Edmundston, NB in about 5 1/2 hours. It's a great place to stretch your legs and grab a bite. You'll be at the Confederation Bridge about 5 hours later. You'll pass Moncton about an hour sooner, so that's another good spot for a stop. Once you cross the bridge (and get past the Anne of Green Gables gift shops), it's about an hour to Charlottetown.

There are a few places I'd suggest stopping along the way (Hopewell Rocks, for example), so depending on what sort of flexibility you have in your schedule, you can make a really nice road trip out of it.