My parents in law live on the edge of the eifel. Have done an uplift day at hurtgenwald (back protector compulsory) which was good but not worth the trip in itself. Have also been to Winterberg a couple of times, bit further out but worth it for a weekend, the runs aren’t long but the lift is really fast so you can get loads of runs in. Did a lads trip last summer and everyone liked it. Accommodation is dirt cheap in the summer too.
Haven’t been trail riding as such, as others have said access is an issue but have seen a few groups of enduro’d up locals dropping into some really sweet looking steep natural trails. FIL told me they shouldn’t have been on those trails but no one was stopping them. That said as tourists you might get shouted at, Germans aren’t usually backward in letting you know what they think.