What the US didn't do in space since the end of Apollo:
Put a human on the surface of another planet.
What the US did do in space since the end of Apollo:
Place a variety of advanced telescopes in space
Sent fly-by missions to every planet.
Put orbiters around Earth, the Moon, Venus, Mars, Jupiter and Saturn.
Put landers in or on Venus, Mars, Jupiter, and Titan.
Mapped numerous bodies in the Solar System.
Discovered what appear to be a number of oceans on other worlds.
[I was surprised to encounter an article from the 1970s that
made it clear the author wasn't sure if the Galilean moons were rocky
Carried out a long term examination of the sun from space.
Sent fly-by missions to a number of asteroids and comets.
Destructively explored the interior of an asteroid.
Sent a high-speed mission to the first known dwarf planet, Pluto,
and points beyond.
Explored the borders of the heliopause.
That's just off the top of my head.
- What did I miss?