To get close to North Korea, take a 30- to 40-minute boat cruise from the tour-boat piers on either side of the border bridges. You have to wait for boats to fill up with passengers (on average 30 minutes), but the pace allows you to take more in than on the smaller, splashy speedboats. In summer you might see kids splashing in the river, or fishermen and boat crews moored on the other side.
Touts sell similar tours for ¥60 that bundle tourists on buses to a pier 40 minutes away. It's only good value if you ask to be let off at Tiger Mountain Great Wall on the way back.