The standard solution is a neutral density (ND) filter, which cuts light so you can use a much longer shutter speed in daylight. A variable ND can work, though quality matters because cheaper ones may affect image quality.

Other practical options are to shoot when ambient light is lower, such as at dawn, dusk, or even by moonlight.

A pinhole-style lens cap can reduce light, but it’s usually not a great substitute for an ND filter. Very small openings introduce strong diffraction, reducing sharpness, and they behave like a pinhole camera with very large depth of field and a different look.

Another alternative is to shoot a sequence of shorter exposures and average them later in editing to simulate the smoothing effect of a long exposure.

HDR is mainly for extending dynamic range, not for creating motion blur, so it’s not the right tool for this specific effect.