If you want an easy hike from the center of Unzen, I would recommend hiking Mount Kinugasayama.
Before starting the hike, you can get a physical copy of the map of the hike that also include information on the local fauna and flora, it is available at the Unzen Visitor Center.
If you start your walk from the hell, you can first pass by the Old Hachiman Jigoku, that used to be an active area with steam coming out of the ground too, then continue to Gensei Marsh, a marsh land that is the final recovery stage from the volcanic activity of the Unzen hell, that is slowly moving east.
Not far from it is the start of the hiking trail. It is quite easy to follow as you can see the remains of a paved path that clearly show years of use and natural degradation. Despite the paving being mostly broken, there is no obstacle in the way, you just have to watch your steps as there is rubble, small branches ans possibly some loose rocks in some parts of the path, but nothing that make the walk much difficult. The incline is also gentle and should not be much of a problem unless you are really out of shape or have mobility issue.
Once on top, you can have some view on the Tachibana bay as well as he surrounding mountains. On the way back, you will reach Shirakumo Pond and the nearby campsite and your hike will almost be done.
In total, the whole hike is advertised as taking around 1h40 and. If you are looking for an easy hike while in Unzen, this would be my recommendation, but for a bit more steep hike, you would possibly prefer to go hike Mount Yadake, that is also accessible on foot from Unzen.