What to do about massive slacking?

My dwarves have suddenly stopped working... to the tune of 30-40 idlers. I feel like I've got plenty of tasks, but obviously not enough. What's some good busywork I can set my dwarves to doing.

First, make sure your dwarves' labor preferences allow them to work those jobs.

Second, make sure you have the tools for those jobs. (It's hard to chop trees without an axe)

Third, make sure your lazy dwarves aren't new immigrants. (New immigrants are lazy)

If after all that, you still need busywork, some good busy work includes:

1. Engraving - smoothed/engraved rooms make dwarves happy!
2. Military training - a trained dwarf is much more likely to survive than untrained militia
3. Herbalism - if your map is safe, gather more plants and seeds to trade, cook, or brew booze with.
4. Cooking - turn boring food into lavish meals!

Long-term projects to utilize your work force might include:

1. Constructing a megaproject.
2. Starting up a new industry.

In addition to what @Arkive posted, I also like to fiddle with my stockpile management pretty closely, which can queue up quite a few hauling jobs pretty quickly.