Skyrim Other Story lines?

I've completed the main story line and defeated Alduin once and for all, however now I am stuck for things to do.

I have done the whole story line that allows me to become the archmage of the college of winterhold, also I have completed the thieves guild and a few other things, including becoming a werewolf.....

Bit stuck and can't think of anything to do?

Replys appreciated....
Thanks.

Do stuff for the Dark Brotherhood

That will be assassinations yes?

you could join the compainions in whiterun

Where do I join the dark brotherhood?

companions ill give a go, thank you.