The Donghae Coast (Korean: 동해 연안) is a region of North Korea stretching along the the entire west coast of the North Korean part of the peninsular. It encompass the provinces of North and South Hamgyong, as well as Kangwon and the foreigner-accessible Kŭmgangsan and Rason regions.
Donghae is the Korean name for the Sea of Japan, Both Korea's object to the name, as something imposed on them under the Japanese occupation. The coast stretches for about 800 kilometres north-west from the border with South Korea to the Russian border at Tumangan/Khasan. As the site of North Korea's latest nuclear test (near Kimchaek) it should come as no surprise that travelling around here independently, or even just travelling, is not going to be the most popular adventure you could embark on. That being said, Rason to north and the Kŭmgangsan Tourist Region to the south are both feasible destinations, and bookable with several Russian and South Korean tour groups respectively.
There is occasional flights to Wonsan and Chongjin with none other than the "reputable" Air Koryo  using a fleet of veteran soviet era planes, that has been banned from flying nearly anywhere outside North Korea. Due to chronic fuel shortages you need be a high ranking North Korean official to book a spot on those flights - if you are reading this, chances are you aren't one, and so you are at the mercy of the whims of your travel agent.
Chilbosan, Chongjin and Namyang may be approached from China.
Slightly more probable is catching a train, there is a railway running along the entire coast connecting to the Russian railway at the border town of Khasan. From Khasan it's possible to book trips as far as Rason through Russian Tour companies, since this is a special economic zone. Through service to Pyongyang via the Donghae coast is not available to foreigners, but it was done by an Austrian and Swiss guys in 2008, causing quite a stir, and initiation of procedures to ensure it doesn't happen again.