i just did the museum of history's 'quest', i went to underworld and i talked to crowley but he didn't have anything interesting to say. i'm not familiar with Oasis, the water caravans in rivet city, nor the Pitt. I have done the fire ant quest in Greyditch.
Museum of History has a small number of linked side quests, Crowley is one of them and he has a degree of replayability which makes him fun in later playthroughs as well. For me the main side quest linked to the museum of history is the 'Temple of the Union' (I may not have the quest name totally right though). You need to find the temple and speak with the leader there and a quest unfolds. Oh and try to find Oasis
Some other suggestions, sorry for duplicating if others have already said and you may have already done some of these.....
- Trouble on the Homefront (check your radio for new frequencies near Megaton).
- Find Agatha's house and speak with her
- Nuka Cola Challenge
- If evil speak to Grouse first and do his 'stuff' then later Euology (this is at Paradise Falls) to see if he has any special requests/quests for you
- Visit Takohoma Park (need to get there via metro's.......
- Explore the entire metro network!
I found metro's hard work in navigating to places I couldn't normally get to when I was learning the game, it's worthwhile spending some time exploring these and seeing where they all lead to and where the dead ends are etc.
If your seeking to gain XP quickly then any DLC should help, I found Mothership Zeta levelled me up a lot but there's plenty of xp to be gained in the other DLC's as well.