What are some safe devices to rapidly deliver small, light physical objects to others at a long distance but still within sight?
What tech level?
A drone is the obvious choice.
With medieval tech, it gets more complicated...
A trained falcon is possible. Or a messenger pigeon.
An unarmored rider on a fresh horse will get to anywhere within arrow distance in half a minute, too.
If you want to transmit just a message, not physical objects, use flags, drums, whistling, a bugle, or smoke signals.
Or get creative: Drill a 1/2" hole into an egg shell, drain the content. Put your rolled-up message inside.
Fill up with sand for weight, cover the hole with a wax seal.
Then use a sling to shoot your messegg.
I'm thinking somewhere between medieval and below a drone. Also delivering a physical object, so not a message that could be conveyed alternatively, and without an intermediary like birds or riders.
Really enjoy the creative slingegg idea!
A looped fishing line between two locations so you can tie things and pull or push from either side. But you would still have to shoot an arrow with the line attached.
between medieval and below a drone
Base ball and bat, or tennis ball and racket
A well designed paper plane
Pneumatic Tubes come to mind.
Or a simple zipline with a motor/crank on side to retract the cart when going uphill.
edit: even simpler, a clothesline.
What's wrong with semaphore?
If you can see each other, just use flags to let each other know what's going on.
A cheap laser pointer and Morse code would also work to send a message.
Nothing at all, just seeing what people think of when asked this.
However the real fun to my mind in this question is not related to delivering a message, but the method of delivering an object.
The thing is that you haven't explained why the messages are being sent. Is it the floor of the stock exchange or a raging battle? Time of day/night matters, too.
There's a tendency in fantasy books to over-complicate things for the sake of world building. People have been arguing about the lack of safety rails on the Death Star for decades.
Unless it's vital to the plot I'd say stick with a tried and true tech.
That said, you could throw somethign the size of a coconut pretty far with a sling or some form of catapault, and adding a bright colored ribbon would look good and you could give the colors meanings; like white for surrender or blood red for an individual challenge.
Nerf gun.
How safe are we talking? I'm thinking cesta punta or tennis racket.
Homing pigeon.
I'm not sure how practical that really is. They actually have to have many homing pigeons carry the same exact message, on the assumption that many of them won't actually go where they expected.
They might change course due to weather conditions or whatever reason, or they could just up and die, either by predators, human weapons or even human traps.
And that also poses a risk, with so many homing pigeons carrying the same message, then unless the message is extremely well encrypted, it still becomes a security risk if even one of those pigeons somehow gets captured by enemy forces.
Nine year old children.
I can think of one thing that can launch a 90kg projectile up to 300 meters
What's the thing?
https://www.wikihow.com/Build-a-Trebuchet