The city has an old-fashioned pedestrian-friendly shopping district in the downtown area. It also has a generic retail strip behind parking lots along the highway, with all the same stores you've seen in the strip of every other city in the U.S.
Fresh fish from Thill's Fishery, on the lakeshore near downtown.
Frozen pasties from Jean Kays to take home.
Trillium Wine if you're not from the area. It's an excellent Michigan Chardonnay.