These wonders will come in many forms and might be either natural, magical or created. The only requirements will be their great renown and awesome spectacle. More likely than not there will be some dungeon or adventure to be found nearby, but mearly traveling to look at these famous destinations will also be rewarded.
I'll write up a short but colorful description for each wonder, to be read aloud when the PC's see it for the first time. Then each character who gazes upon the wonder will be awarded some XP.
I'm thinking a reward of 1d10 x 100xp sounds good, but maybe I'll use different dice for different wonders. I like the idea of including a random roll to represent the subjective taste of a person to find certain wonders more impressive than others. Each wonder will probably also have bonuses for certain species or circumstances.
I had thought it would be easier to imagine a bunch of unique wonders, but right now I can only seem to think of wonders from history or literature. I'm not above stealing ideas, so I'll probably come up with something based upon each of the pictures I've included here. I'll have to wait for inspiration to strike to get some truly unique wonders, maybe I should focus on wonders made with magic? .....or wonders of magic, hmmm.
I suppose I should also lay the groundwork for an XP reward for creating wonders. Because (after they find a few of these) the chance the PC's will go out of their way to build a new wonder is about 100%. Maybe, 1xp per 1gp spent on the construction? It would probably just be easiest to have them gain a level once the wonder is completed. The wonder will have to be impressive enough that people would write poetry and sings songs about it forever... obviously.