I usually do a sort of caravan run
I'll kill fiends, powder gangers, super mutants, assassinate Ranger Veterans, etc.
Take their stuff, repair it, then try and sell the stuff to get every cap in the waste.
It can be pretty satisfying knowing you have the caps of every vendor in the waste.
Other than that I'll usually make a save then see if I can take on everyone in the Strip, or at Camp Golf or Forlorn Hope.
That late it the game its not much, but it can kill a few hours if you need it.
Oh, also, going on an acheivement run can kill some time to
Otherwise you can always pop in another game to kill some time