I did the Zhongdian-Xiangcheng-Litang-Kangding route back in 2009 and loved it. This was also a time that much of Aba was closed off to foreigners - I remember a couple of check points where my passport was checked. Anyway - this might be an option for you - my inclination is that this route tends to remain open when other Tibetan areas are closed. I didn't do much hiking on this route, other than a quick hiked up Paomashan in Kangding - Gonggashan in Sichuan is an area that would almost definitely remain open to you - and probably has excellent hiking opportunities (I didn't quite make it down there because I ran out of time on that trip).

As for other sights around Changbaishan - the entire North Korean border region seems interesting to me - Yanbian, the Koguryo tombs (a world heritage site) and scenery in Huanren, Liaoning, Dandong.