Central Circuit : Noboribetsu
Noboribetsu actually comprises the nearby towns of Noboribetsu, Horobetsu and Washibetsu. However, for most visitors the main target is the hot spring village of Noboribetsu Onsen (登別温泉), 6 km inland from Noboribetsu itself. The vast majority of (Japanese) tourists are drawn to the 11 types of water found in the local hot springs. Water contains sulfur and iron.
Noboribetsu is located on the JR Muroran Line between Oshamambe and Tomakomai.
From Sapporo, Noboribetsu can be reached by limited express in about one hour at a cost of ¥4160. The run from Hakodate to Noboribetsu is about 2 1/4 hours at a cost of ¥6500.
Noboribetsu is also served by overnight trains: The Cassiopeia and Hokutosei trains to and from Ueno Station in Tokyo, the Twilight Express to and from Kyoto and Osaka in Kansai, and the Hamanasu Sapporo-Aomori express (the Aomori-Sapporo Hamanasu does not stop in Noboribetsu).
If you're looking for a cheaper alternative than the train, Chuo (中央) bus and Dounan (道南) bus offer cheap, ¥1900 yen (one-way) trips to Noboribetsu in about 1 hour and 40 minutes. Boarding happens at Hakodate Station and Chuo bus terminal [中央バスターミナル), and schedules can be found here (relevant station 登別): 
The main sights of Noboribetsu Onsen can be seen easily on foot. Sensible footwear is highly advisable as this area gets a lot of snow. Some of the paths around Hell Valley can be impassable even in Spring.
Get wet! The area boasts a huge number of hot spring resorts in all shapes and sizes. The majority are within walking distance from the bus station. If staying overnight it is de rigeur to wander around town dressed in yukata.
The town has plenty of souvenir shops and there is an abundance of demon related paraphernalia to take home. Bath salts are also very popular. Keep an eye out for locally made beer, the cans come in blue or red and have a demon motif.