This sprang out of a series of comments on Google+ on one of my posts. Basically, I said to offer rewarding hooks to the PCs if you want them to try new things. But the idea was at least partly rejected as being wasted work - why prep things no one wants?
Thinking it over I realize that fundamentally with a sandbox type of play, and with repetitive use of the same play area (game to game, session to session, campaign to campaign) that to my mind there is no wasted prep. No prep is unnecessary prep, if you know what to do with it when it gets "wasted." Even if the players don't want to interact with the stuff now (or "ever"), it's not wasted if you know what you can use it for.
I think I feel this way because of the style of game I run - a sandboxy megadungeon. The PCs keep coming back to the same place, and if they don't go left today they might do it tomorrow. If they don't run down a tasty rumor this session, they might ten sessions from now. And if they do go too far in one direction I wasn't ready for, well, I have tool to quickly fill in the gaps. And I have clever ways to stall them without discouraging them from keeping on in that direction.
But what if the prep is for someplace the PCs never go, or for interactions the PCs don't intend to have? Or if you overdid the prep a bit, and went beyond a useful work:reward ratio. You started naming each goblin in the random patrol they might not encounter, detailed all the stock in the stockroom at the inn they'll never visit, or drew up dungeons in places they have no intention of going. Now what?
Repurpose - the first thing you can do is re-purpose the details. Find another way to use that prep.
Names for the goblin patrol? You can re-use those names when they capture a bunch of goblins.
Stock at the inn? Ready made list for the next inn they go in. Or the next stockroom they happen to duck into and search.
Dungeon all drawn up? File the names off (if they matter) and place it where they are going.
Diplomatic mission all detailed, but the PCs never meet them? This can suddenly become stuff found on the corpses of the dead diplomats when the PCs stumble across them.
Reuse - lot of prep is reusable prep, either in whole or in part. Monsters, treasures, NPCs, etc. - all of them are there to use again and again.
Dungeons can be re-stocked.
Monster encounters can be bypassed now, but then turn out to have important stuff people need later.
Diplomatic mission details can be re-used when the King keeps sending the same folks out to do his negotiating. They might not get met in play, but they can provide a steady background to setting by always being around and name-dropped by the GM in rumors and news.
Re-play - Even more simply, just keep running games on the same world. If every fantasy game you play is in Greyhawk, then "wasted" prep on the border wars in the region where you dungeon-bashing party simply raided dungeons is still good prep. It's now a rich background of events, with player-driven causes and ends, in a play area you can re-use with different players or the same ones playing a different style.
One of my gamers once played in a Rolemaster game set in a world that had been cataclysmically altered by a previous group of PCs. What made the world so fascinating for him to play (and me to hear about) was the trashed world had been trashed by the GM's previous group. Nothing felt forced, and the drive to investigate what the previous group did fed game play.
As low-detail as my own Felltower game's gameworld is, little bits keep emerging that make it a good place to set another Dungeon Fantasy game, or even a larger fantasy game if I ever feel like running one. So the little scattered stub ends that say "fill me in if someone cares to do this" make for a potentially richer play area sometime later. And even if they don't, the fact that these little bits (the cities of Cashamash and Molotov, the keep on the borderlands, names of holy inquisitors, hints of other dungeons) make the world feel deeper, richer, and endless. And because I can re-use and re-purpose and replay, it really is all of those things.
That's why I feel like no prep is wasted prep, with this approach to the game.