Off the top of my head, and in no particular order:

-Chengde
-Goose and Duck Ranch
-beijinghikers.com
-Qinghuangdao (sp)
-Jinan (there's really not much to see there, but the train is pretty convenient; it is fairly close to Mt. Tai)
-Commune by the Great Wall (Kempinski)
hot spring resorts (Jiuhuashanzhuang in Changping is the only true resort I've been to, but I've heard there are several others)
-Shenyang (surprisingly nice)
-Dalian (maybe a bit too far for a weekend train ride)
-Taiyuan (probably the least appealing option on this list, but I'm including it because it is reasonably close)