How about a solar powered flashlight? Might be able to last for a long, long time, and perhaps change the nature of industrialization down the road.
They’d have to have knowhow of developing the film and keeping it unexposed (i.e. not opening the film compartment.)
An instant Polaroid avoids these issues. I imagine, they’d inadvertently click the button, and the polaroid would spit out the white, still-developing square picture, exposed to the lovely Mediterranean sun, as it develops magically before their eyes. Perhaps a blurred image at first, but the Greeks were no dummies at that time. I think they’d pick up what the device did in no time.
Once they got it, who know’s what they might snap pics of. Caesar even? This clear Christian miracle might even be preserved by the Pope at the time (Benedict VIII no less), and end up in the archives of the Vatican.
And hence, Catholicism would take an unlikely scientific bent, averting the dark ages! It could happen.
Some kind of small hologram device that will display an actor pantomiming the time travel rules so they don’t fuck up and send something back in the box that’s not allowed by Czarcasm.
Like Rufus playing the Wyld Stallyns Air Guitar Salute?
Care to complete the request of the OP?
What country would best be able to figure out how it works?
I wasn’t wrong at all…dude, and apparently it’s not that simple if the premise is so shakey that you have to keep adding rules.
And thank you for pointing out that no one is forcing me to play, you must be fun at parties.
If you just want to fuck with them, there are countless things you can send back, but if you want to accelerate technological development, there are only two types of things you can send:
- something that the technology exists in their time to make, but the idea hasn’t been come up with yet, or
- something that they’ve had the idea for, and have most of the technology to make, but they just need a small clue to develop a missing manufacturing technology.
I can’t think of anything off the top of my head that will fit either of those two conditions, but even if I could, it probably wouldn’t advance any development by more than a hundred years, if even that, over our history.
A better way to make a difference would be to find a war where the winning side came up with a key war winning technology that the losers immediately understood and began to copy, but too late to make a difference, or the losing side came up with an idea that could have won, but they came up with it too late. I can’t think of an example from 1000 years ago, but a modern example would be the tank in WWI. If the Germans had come up with it a year before tanks were actually developed, they might have won before the US got involved.
Absolute conquest of the civilized world, of course 
A walkman with all logos and markings removed, with only one cassette preloaded, and with only one song recorded and rewound, sent to Germany.
As soon as someone hit the “Play” button, Rick Astley’s Never Gonna Give You Up would blair across the royal courtyards.
Yeh, I just Rick-Rolled Henry II, King of Germany.
According to the revised rules, no recorded words…but I’ll allow non-vocal music. And probably send back “Music Box Dancer”.
Again. The Greeks. Trust no one else. The Mongols ruled China at the time.
No wait, I’m wrong, Genghis Khan didn’t show up on the scene until the 1200s. Maybe China, then.
Let’s say England. Not so much that they’d figure it out first, but trying to nudge the industrial idea of ‘always on’ to ‘available when you need it’.
It seems like the steam engine (and it’s descendants) powered the constant need for energy we have now, and it was a bit of ideological thinking on my part. The ‘renewable energy’ resources were there before, with windmills and such, and maybe this would help demonstrate we have come all the way back around to thinking along those lines.
Given the criteria in the OP I will go on record saying I don’t think there is anything you could possibly send that would make any difference in history other than possibly creating a puzzle for present day scientists when they dig whatever it is up and wonder how it got there.
How about a 2’x2’x2’ cube of uranium to the Vatican?
For what it’s worth, my mention of “an Uzi” was just to illustrate the “kill my ancestors” paradox and not a real decision. I’m sticking with my llama-wagon choice.
How about this compound bow to the Chinese?
A pair of uranium cymbals.
Because the Roman Catholic Church is heavily into cymbalism?