We only spent a day here but still took in quite a few things. First off, stop at the currency exchange in the little strip mall right before the border crossing and get some cash. The border crossing is no problem - they didn't even ask us for I.D. I guess we weren't suspicious looking. Thunder Bay is maybe 45 minutes beyond Grand Portage. The town isn't all that different than an American small city. We went to the amethyst mine about a half hour northeast of Thunder Bay on 11/17, got a brief tour, and then dug for our own amethysts. We took home a large bag of amethysts for a few dollars and dumped them in our fish tank.
Another thing to do is go shopping at the mall. The prices are cheap compared to U.S. prices and a lot of the shops are different than here (but a lot are the same, such as Sears and Wal-Mart.)
We also drove up to the Mt. McKay scenic lookout (Take Hwy. 61B off Hwy. 61 and follow the signs). There is a short drive up the mountain through a birch forest, beautiful in fall, but you have to stop and pay a couple dollars to go up. At the top you get to overlook the city and the harbor.
Also, if you aren't sick of waterfalls by this time, check out Kakebeka Falls Provincial Park. It is 15 - 20 miles west of the city on 11/17. We didn't have time for Old Fort William, but have heard it's very good. It is the world's largest reconstruction of an early 1800's fur trading post with costumed staff reenacting daily life.