Winter is a great time to visit here especially if your into skiing. The reason is clear in winter the Hohe Meissner is a skiing paradise, with ski lifts, cross country ski tracks, ski slopes and ski jump facilities.
If your into relaxing visit Bad Sooden-Allendorf and try a pure air spa as they are everywhere and not overly expensive.
Museums to visit
Salt Museum (Bad Sooden-Allendorf)
Glass and Ceramics Museum (Grossalmerode)
Border Museum (Bad Sooden-Allendorf)
Ethnology Museum (Witzenhausen)
If water sports are what your looking for try a visit to Lake Werratal, the lake is host to a large variety of water sports.