The Kushiro line runs from here to Abashiri, on the north (three hours, only local trains). This line passes through two National Parks with volcanoes, springs and lakes. Usually the train has to reduce its speed because the wild deers decided to take a shortcut through the railway. The natural experience for most urban Japaneses on holidays.
See the travelogue!
If you're in Kushiro in winter and looking for a chance to get outdoors and see the marshland you can rent x-country skiis for free from the Onnenai Visitor Centre. Take the road to Tsurui (North of town) and when you see the big Marshland Observatory continue along for 5km. On the right is a largish car park, and steps leading down to the Onnenai Visiter Centre. There are board walks around the Marsh and you ski on these.