If you press ctrl+z again, no matter what civ you will 'rediscover' those natural wonders, and regain the happiness benefit (the game will say you have -7 natural wonders yet to discover, say). If you are spain, you will get a couple of thousand gold from this. Simply press ctrl+z (the first time you press it it will reveal the whole map, introduce you to all city states, all empires, reveal all resources and discover all natural wonders. Unlimited happiness applies to all civs, while unlimited gold only applies to Spain.
The following (as far as I have tested) works on all difficulties at any point in a game with one or more natural wonders, and I cannot find it mentioned elsewhere on the forum.
