Check out Washington Trailblazer App. You can plug in exactly what you want length, conditions, views, location and it will give you a list. There are also usually condition updates by hikers.

I am going at the end of October. I believe December isn't a good month for hiking in Seattle plus you will be missing out on the flora of Fall. Anyway Skyline trail and Mount Fremont Lookout trail are the best recommendations!!

December in the mountains here will be full on winter so be prepared with winter gear, clothing, footwear, snowshoes, etc. as well as car that can drive in the snow. Trails look different in winter too and some trails/roads tk access trails here are closed, just wanted to give a heads up! But I think there’s something absolutely magical and even more beautiful about winter hikes ✨☃️