For some surprising reason, the sleepy former mining town of Røros gets 1 Mio visitors each year. When I came, the main road was busy and the parking lot full with caravans and cars. The main street is lined with cafes and shops, as in every other tourist town.
The obvious comparison is with Falun, another mining town in Scandinavia. Falun features a big hole and that is the striking feature of the mine. Røros, meanwhile, is more about the town around the mine, so I think these two sites are distinct. All the colorful wooden houses reminded me more of Rauma than Falun. And I enjoyed my visit significantly more than visiting yet another central European mining town.
The dominating features of the town are the church, the smelting house, the poor worker cottages and the slag heap. The smelting house now houses the Roros museum and is the starting point of guided tours. I would discourage visiting the museum, though, as only one room has been kept in an original state. All other rooms have been repurposed for the exhibition.
Best part to me was just wandering the small streets.
Getting There
As pointed out by Els, the easiest option is to do a day trip from Trondheim. Train leaves Trondheim main station around 9:30 and arrives in Røros at 12:15h. Return trip is at 15:30 or 16:30 depending on the weekday. Be sure to check the schedule for the precise day you intend to travel and book in advance as you need a reservation and this is a small train.
Three to four hours is more than enough to take in the town and have lunch and coffee. But it's not enough to explore the surrounding components by public transport as they are rather far away.
An alternative to the day trip from Trondheim is to use Røros as a stopover when travelling between Oslo and Trondheim. The travel time to/from Oslo is five hours. This may allow for a cheaper flight.