The Chicago Architecture Foundation's riverboat cruise, departing from the Michigan Avenue bridge on the main branch of the Chicago River, is the single best introduction to the city's architectural history. The 90-minute tour passes approximately 50 buildings visible from the river — including many mid-century modernist towers not easily appreciated from street level — with professional architectural commentary.
Reservations are strongly recommended in summer; same-day tickets are sometimes available. The tour is expensive by Chicago standards but provides context that changes how you read the city's skyline. Multiple tours run daily from April through November; the evening departure captures the skyline at a different light and is worth the slight premium. Michigan Avenue at Wacker.