Yeah a huge alien ice spaceship.

Actually, a large part of a spaceship capable of traveling near c would be comprised of ice, as a shield from relativistic particles.

https://www.fourmilab.ch/cship/craft.html

It slices! It dices! It's a shield! It's reaction mass! It's fuel! It's potable water!

Interested in how this design decelerates

I thought of that too. It sure would be exciting if it turned out to be something artificial.

However, I agree with you that it's far more likely that it's something perfectly natural like ice. Not quite as interesting.

I really want the New Horizons probe to whiz past Pluto and find a giant "WE WUZ HERE" scrawled across the surface.

Run, don't walk, and read Clifford Simak's _Construction Shack_.

There's a pretty crappy but readable copy here: http://lingualeo.com/pt/jungle/construction-shack-by-cliffor...

Is it supposed to end in the middle of a quoted sentence? (I mean, I can imagine it ending that way, but it does seem kind of abrupt.)

At 'the bunglers'? Yes, that's correct.

Or maybe, we will find some prothean ruins on Ceres and Pluto is a mass relay... ;)

Or our space junk.

our space junk is stuck orbiting the Earth.