RheinRiff runs a CityWave setup inside what its owners bill as the world's largest indoor surfing hall, tucked into Düsseldorf proper rather than out in the suburbs. The nine-meter-wide wave splits into sessions for both beginners and advanced surfers, so you can show up cold and still get useful reps in. Expect to pay to surf, and expect company — the stadium-style layout around the wave makes it a natural spot for contests, with a beach club, volleyball courts, and a surf-skate course keeping things lively even when you're not in the water. It's a solid year-round option if you want consistent, bookable reps without waiting on river flow.