Stats, definitely the stats.
Ideas take no time at all, I always have more ideas than I have time for.
Dungeons, towns, etc? They take some time, but I sometimes make maps just for the heck of it, pulling them out when I need them Sometimes I even stat out towns ahead of time.
Creatures? There are a lot of very nice database programs for critters out there.
Traps & Hazards? Easy, again, more than I can use, some written down, others hovering in DM Space, waiting.
Treasure? If I get stuck I use Jamis Bucks treasure generator, otherwise I just keep an eye on the totals for treasure.
Then come the bloody darned stats... I don't like using pregenerated NPC for anything except very minor NPCs, so I have to do anything major by hand. Thank the gods for PCGen, it has made my life so much easier in regards to NPC statting. But it doen't have all the books that I use so for anything that is campaign specific out comes the word processor.
The Auld Grump