Omoshiroyama is the in Yamagata Prefecture and one stop before Yamadera on the Senzan Line and about 50 minutes from Sendai. Omoshiroyama Kogen has a nice hiking trail that runs next to a river. If you want to go to Yamadera but want to do some hiking as well, Omishiroyama Kogen is a good place to get off.
The hiking trail is about a 45 minute to an hour walk (assuming you are not stopping to take photographs–it took me about an hour and half taking photographs), but then after the trail runs out you have to walk another 45 minutes or an hour to get to Yamadera. Also there is not much as far as signs pointing where to go, so don’t go by yourself or don’t go if you are short on time and want to make it to Yamadera.
The hiking trail is really quite beautiful, especially during fall, as you pass by waterfalls, through a gorge and over bridges; however at some points the route can be a little slippery and very narrow. Once you get to the end of the trail you will go up some steps which will take you to a pathway which leads to a road. Once you get to the road, turn right (downhill) and there shoot be signs coming up soon to verify you are going towards Yamadera. Again I do not recommend going by yourself.
The above photo was taken after the hiking trail and walking along the road towards Yamadera.