Photo: GetYourGuideThe best-selling boat trip of Frankfurt glides along the Main past the skyline, the old town and the museum riverbank, with commentary picking out the sights on both banks. It is an inexpensive, relaxing way to see the city from the water in under two hours. Sit on the open upper deck if the weather allows, aim for a late-afternoon sailing for warm light on the towers, and pair it with a riverside stroll afterwards.