Shiraito no Taki (White Thread Falls) near Karuizawa is a bit too touristy and crowded, but the rest of the hiking trail from Mt. Asama down to Karuizawa town is almost completely empty. This was a long exposure done with a three-stop neutral density filter (the lens was stopped down to f/10 but with the ND filter the exposure was equivalent to f/28). I have seen many people stopping down to these kinds of apertures directly to get long exposures, but using extremely small apertures is also prone to other problems. If you don't have an ND filter (they're fairly cheap), you can also use a polarizing filter which blocks as much light as an ND4.

Canon EOS 300D, EF 17-40mm f/4 L @ 25mm, f/10 + ND8, 0.6 sec, ISO 100