Float the Rhine through the medieval Old Town with a Wickelfisch dry-bag
Seal your clothes and phone inside a fish-shaped dry bag, slip into the Rhine upstream by the Tinguely Museum, and let the strong current carry you a gentle ~3 km past the red-sandstone Minster and the Old Town to the Johanniterbrücke. It's the quintessential Basel summer ritual locals do after work, and no other European city lets you swim through its historic core like this. Only do it as a confident swimmer with at least one buddy; the current is fast.