Is it a month one-way or round-trip?
Toronto would be a nice stop. It's a big city to explore but you'd probably enjoy walking down Yonge St. and along the lake shore. Take the ferry out to Toronto Island Park. You can also visit the CN Tower. The city has a lot of quirky neighborhoods too, like Queen Street West.
Ottawa is a lot smaller than Toronto but it's also a very nice city. The Canadian Parliament buildings are really stunning. Maybe you could take a tour?
Montreal is very cosmopolitan and interesting. The Rue St-Denis (St. Denis street) in the Quartier Latin (Latin Quarter) is a great place to eat at night, especially if you get a table outside and do some people-watching. Mont Royal Park is huge and great for walking, hiking, biking, etc. The St. Joseph Oratory is a massive modern cathedral with really cool architecture perched high up on a hillside. Old Montreal / the Old Port is another lovely part of the city - check out the Notre Dame Basilica. It costs about 5 dollars to go in but you won't regret it. The interior is absolutely beautiful.
Quebec City is one of the most beautiful cities in North America (and probably the world). The Old City is amazing. There's a nice hillside park that overlooks the Chateau Frontenac and the St. Lawrence River... one of my favorite places to lounge on a sunny day.
Instead of going straight to Ft. Kent from Quebec, you might consider following the northern bank of the St. Lawrence up to Tadoussac at the mouth of the Saguenay River. This is a beautiful area and it's great for whale-watching. Then you can take the ferry to Rivière-du-Loup and head down to Ft. Kent from there.
If you're taking a different route back or if you have time to spare between Montreal and Quebec City, I would really recommend going down into northern Vermont and New Hampshire. The hilly and mountainous terrain will be a nice change from the mostly flat landscape along the rest of your journey. A Google search will give you any number of scenic country drives in northern VT, but it's hard to wrong no matter what route you choose.
Crossing over into NH, the White Mountains offer some fantastic rides, especially through Franconia Notch State Park (stop and walk through the Flume Gorge) and along the Kanacamagus Highway (a lot of great turn-offs here for views and picnic areas). If you're not afraid of heights, drive up the Mt. Washington auto road and enjoy the limitless views on a clear day. You might consider a drive down into the Lakes Region, through Sandwich, Meredith, and Weirs Beach - famous for its annual Bike Week - looping around Lake Winnepesaukee to Wolfeboro and towards the Castle in the Clouds.
I hope that gives you some ideas!

When are you leaving?