I've thought about this thread a little more and I now think that most of the suggestions from earlier in the thread (writing, birth control, sanitation, vaccines, etc.) work more to reduce suffering than increase happiness. That distinction might seem like splitting hairs, but I do think it is an important to separate the two. Across time and cultures, I think reducing suffering is a more important goal than increasing happiness, even today, because it means providing basic health, food, shelter, and safety to people. Increasing happiness should be more than that though, it should require a certain joyfulness or hedonism. With that, I think some of the more recent suggestions are better contenders for increasing happiness:
Music
Beer/wine
Dancing
Sex (that is not rigid or restrictive or only for procreation)
Food (not just subsistence, but delicious food that can be enjoyed with family and friends)
Air travel (ability to go anywhere around the world fairly easily and cheaply)