What is good to wish on?
- World Peace
Of course, it would be screwed over by the Genie....
3 wishes per day is a lot. Mechanically speaking (in 3.x), a Wish could grant you an ability score increase. (1 Wish per +1 inherent bonus, they don't stack, so if you want a +3 bonus, you need 3 wishes at once.) That might be one of the most powerful wishes the game rules describe (and wouldn't result in you screwing over by "secondary" effects of the Wish.) It's also pretty boring, overall.
You can raise a dead person, and also replicate spell effects (8th level or so is the limit?). That might serve as guidelines at what might be reasonable to ask for.
I think one of the best wishes you can give away is one that is not egoistic and helps other people. If World Peace is too much, you might help a local village, maybe guarantee that it will never have a bad harvest and that they will never grow ill.
It could be something related to the module story. Maybe a wish is required to open a portal, or to get an item that can be used to defeat the villain or one of his powerful monsters.
You could ask for a powerful magical item. A Longsword +5 or a Ring of Invisibility. (The Wish spell also has guidelines on how powerful the item can be.)
The question is - is it about "power-gaming" or "narrative impact"?
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If you're not talking about 3.x and the Wish spell, consider how the wish is realized. For example, maybe the Genie can offer to fulfill you a wish, but he can only do what is in his own power. So if you demand riches, the Genie will have to steal the money or become a succesful banker (or help you become one.) Or he has to locate a dragon hoard and help you find it and defeat its owner. (Or he guides you there when the owner is away and you will have to deal with the consequences alone)