Langtang valley trek, Gosainkunda trek and Helambu cultural trek are such that they can be done independently as well as they can be combined together to make a single trek. Another option is,  Gosainkunda trek can be combined with Langtang valley trek or Helambu trek. The third option is crossing from Langtang valley to Helambu via Ganja la pass (5106 m). But remember that this is a serious pass needinng technical climbing and not worth the effort.So far the best trek seems to be the Langtang Gosainkunda trek, at least in our opinion.