The hardest one is, I think, the Louvre. It requires 2 works of art and 2 artifacts, all from different eras and civilizations. It also gives one of the largest bonuses (+8). Oddly enough, I sometimes have trouble filling that ones that require multiple artists from the same era and civilization, because I advance in tech too quickly to make a bunch of artworks in the same era.
One trick to get around this is to save Great Artists (or Writers or Musicians) if you know you're about to change eras. The great work is dated from the era it was created, not the era the great person was born. This same trick won't really work for the Great Musician tourism bomb though... the tourism they produce is fixed when they are generated and doesn't scale, so saving GMs from the Medieval or Renaissance era just for a late-game tourism boost would be a waste.