Quest Givers

A classic tool that can be used by a DM is the humble quest giver. You can build an entire campaign around a group that sends your PCs around on quests while you try incredibly hard to think of a dramatic storyline that was happening in the background all along!  Here are a few examples:

1. The Harpers

In with a bullet is D&D's most iconic and least secret 'Secret Society'. These guys and girls are great as they operate EVERYWHERE and have an internal ranking system so your PCs can progress up the ladder giving a sense of achievement and something to work towards. The downsides are that the group has to be good aligned and also experienced players may roll their eyes at you. 

2. The Zhentarim

The Zhentarim posing as the Harpers to forward their own nefarious ends, the group realise they have been co-opted into an evil scheme and try to undo their foolish mistakes. It's your basic good guy inversion storyline, your players will realise that they have powered up the very group they were supposed to get rid of. They will be angry at you... Great stuff! 

3. Re-skin a popular TV show either on purpose or by accident... 

I may have accidentally done this in BOTH of my campaigns. Avatar: The Last Airbender and also Stargate SG1. 

4. Elder Scrolls style Guilds

These can be tailored to your group depending on their class and alignment mix or given out as solo missions if the occasion arrises.

The great thing about quest givers is that you can just go wild with the types of dungeons and missions you can create. There's no need to keep them all the same theme of enemy, architecture or even type of mission. You can run with any silly idea you have and it'll fit the game.