>Transit of Venus in front of the Sun, last time this century. As with a solar eclipse, looking directly at the Sun is dangerous - I am not sure whether there would be much to see, anyway, but specially designed, extremely dark glasses might be used. I am not sure whether welder's glasses are dark enough. And looking through a telescope or binoculars is VERY dangerous - the concentrated light is enough to start a fire. However, if you do have a telescope, an image can be projected on a piece of paper.

That assumes there will not be enough advance in spaceflight to put humans in position to watch a transit any day of the week.